Conclusions on Aluminum Adhesive Bonding Tests

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by GESchwarz, Mar 19, 2009.

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  1. Mar 19, 2009 #1

    GESchwarz

    GESchwarz

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    I’ve done my homework!

    I have compared 4 types of methacrylates and 2 leading epoxies on nearly 180 test coupons. These coupons were prepared in a scientifically controlled manner with detailed notes taken. The coupons were all subjected to accelerated heat aging at 150 deg F and thermal cycling from 20 deg. F. to 150 deg F. for the purpose of arriving somewhere at the bottom of the reliability bathtub failure rate curve. The testing was divided up into 180 degree peel, shock vibration, and lap shear. This work has consumed perhaps 80 hours of planning and execution over the course of the past 5 months.

    I have come to some definite conclusions.

    Preparation:

    Must wipe original finish with acetone or MEK first to achieve a clean surface, then 100% abrade both mating surfaces using a high speed sanding disk or flapper wheel of approximately 100 grit. Smooth or even smooth etched surfaces consistently failed against the abraded surfaces. I personally wouldn't trust a Scotched Brite surface either.

    Freshly abraded surfaces out performed abraded surfaces that had subsequently been phosphoric acid etched and vigorously rinsed with distilled water and a brush then dryed. The adhesives materials just seem to like sticking to fresh virgin aluminum. The etch and rinse process seems to only have acted as a contaminant.

    Application:

    To achieve an even layer of adhesive, trowel the material on using a 24 tpi hacksaw blade as the trowel. Use a coarser blade to apply more material for gap filling, or a finer blade only for very precise fits. Apply the adhesive to both surfaces before mating.






    To prevent excessive squeeze out resulting in spots where there is no more adhesive, two methods can be used.
    • Trowel adhesive to one surface and let it cure prior to a second application to the other surface, then mate. This only works with adhesives that are viscous enough to hold the plowed field texture until setup. Proseal and some methacrylates perform well in this way. Epoxy just flows out smooth.
    • Use some form of micro spacer such as micro balloons or nylon tule netting fabric, which is about a .009” thick, just right for an adhesive bond joint. I used the tule. Be aware that too much micro balloons will displace the adhesive, thereby reducing the strength. The balloons of course have no strength of their own.


    Adhesives:

    Most epoxies and some methacrylates are brittle and are therefore intolerant and fail in the presence of vibration and parent material deformation under load. The thing to look for here is the material’s elongation percentage. Whereas 7% is brittle, which is bad, and 50% is very tough, which is good. An X-acto knife taken to 7% material will make chips to the surface; 50% material can be sliced with moderate force.

    Methacrylates do tend to bond more readily to aluminum than epoxies. Methacrylates do stick pretty good to smooth aluminum, but it sticks even better to the rough texture better, despite what the salesmen tell you. I know, because I've seen the difference with my own eyes.

    Failure Modes:

    There are two types, Interfacial which is bad, and Cohesive which is good. Interfacial is where the adhesive debonds from the aluminum. Cohesive failures are where the fracture passes through the adhesive itself. Interfacial failures always occur at lower load levels than cohesive failures. Interfacial failures are a result of poor surface preparation, vibration, or deformation of the substrate.

    If everything is done reasonably well, the cohesive strength of epoxies and methacrylates far out perform standard AN –3 and –4 rivet spacing schedules. It's the interfacial strength that is the Achilles heel of the brittle epoxies and some methacrylates.

    My standard test coupons bond joints were .75” x 1.5”. When adhesive joints are made per the best practices and materials of that described here, I have been able to shear groups of three AN –4 rivets consistently when using .065” material; and groups of four –3, and groups of two –4 rivets all of which tear through .023” 2024 T3.

    My most recent bonds remain unbroken, as their strength is coming to the limits of my rather ad hoc pull test set up. Can I break a group of four –4 rivets? I don’t really need to. If I can consistently break two at a time, then I have a quality process. It doesn’t matter how strong the maximum strength of your joints can be. What matters is, is how reliable the minimum strength is.

    Incidentally, I found that on thin sheet metal, the rivets would tear through the sheet, and the thicker sheet would shear the rivets; all the while, the tougher adhesive joints held. When joints of dissimilar material thickness were pulled apart in peel, the interfacial failures always occurred on the thinner material, the side that deformed.

    The top performing adhesive I used was Partite 3750 from Parson Adhesives, followed by Extreme 310 and Extreme 5375HS. The brittler stuff was Partite 5140 (which was surprising because they advertised it as being rubberized), and the two epoxies, PTMW ES6228 and Solar EA-0504

    So there you have it, the fruit of my labor over the past 5 months. I am confident that I will be able to skin by plane using Partite 3750 or something similar to it.

    At this point I consider myself somewhat of a self-made expert on this subject. I am however soliciting further light and knowledge, and any gotchya’s that may be awaiting anyone considering adhesives in lieu of rivets for secondary structural attachment.

     
    Last edited: Mar 19, 2009
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  2. Mar 19, 2009 #2

    rtfm

    rtfm

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    Hi,
    This is extremely valuable info - thank you for sharing this with us. Where else on the web can one glean this sort of detailed information - for free?!:)

    This one goes straight into the bookmarks.

    Duncan
     
  3. Mar 19, 2009 #3

    wsimpso1

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    Schwartz,

    In the literature, oxidation of the aluminum at the interface (facilitated by moisture) was cited as the primary failure mode of the joint in long term exposures. What do you know about these joint systems under longterm moisture?

    Billski
     
  4. Mar 19, 2009 #4

    GESchwarz

    GESchwarz

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    Excellent point Billski.

    I did not do any testing on moisture ingress and corrosion. Because I live in Southern California, I never considered it, and for me I don't 'think' it's a problem. Like you say, for corrosion to occur there must be moisture, at a certain level. I would imagine that there is a direct relationship between relative humidity and the rate of corrosion. I got to believe that the adhesive is a fairly effective moisture barrier under all but the worst conditions, such as continuous moisture or salt fog. After all isn't that what paint is for? Certainly paint is an effective moisture barrier. I would think that methacrylate and epoxy are equal or better than paint as a moisture barrier. If any one can shoot down this reasoning, please fire away.
     
  5. Mar 19, 2009 #5

    orion

    orion

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    Nicely done Gary.

    I'd just like to add a couple of minor points. In the case of structural bonds, in aircraft application it's generally important to use a system that does have toughened properties. What this means is that the epoxy has a rubber additive that allows the material and its bonding mechanism to deflect during conditions of shock, preventing catastrophic delamination. The toughened properties also aid the bond integrity in areas of vibration, as you might have near a firewall or near the landing gear.

    When choosing a resin system, it's important to look at all the properties, not just the materials' base strength or simple ASTM shear strength. Some very high strength systems will tend to deliver very poor long-term service. For instance, we're currently evaluating new candidate epoxy systems since the original formulation I chose for our projects is turning out to be rather pricey. The current baseline product is 3M's DP460NS - this is an excellent product but buying it in 400ml cartridges, the cost is coming out to about $730 per gallon.

    As an alternative, I was hoping to get a better price on 5-gallon buckets, which would be applied through a pumping/dispensing system. But to justify the cost of this system (about $10k) the price of the epoxy would have to go down quite a bit. But alas, no such luck - the 5-gallon buckets still ran about $500 per gallon.

    As a result, I've been reviewing about twenty other systems, finally narrowing it down to two. The interesting aspect of the research showed that although some systems had excellent properties under one condition, other conditions resulted in a dramatic drop in bond strength. For instance, one set of epoxies had excellent adhesion and shear properties but a closer look showed that peel resistance was virtually zero. A good peel strength to look for is at or above about 35 pli (pounds per linear inch); an excellent peel strength is over 45 pli and I've seen some that test out to over 60 pli. But this set of high strength epoxy formulations showed peel resistance of only about 4 pli.

    Another issue is that of temperature. There's actually more to this than I care to write about herein but in general, you want to retain as much strength and bond integrity as possible as the temperature goes up. And this issue becomes very critical if you're on the tarmac in the Southwest, heat soaking all day. Under certain conditions, even a white airplane can achieve surface temperatures in excess of 150 deg. F so one must consider the effect that will have on the critical bonds within. The interesting thing here is that composite structures will fare better under these conditions than aluminum ones will simply because the composites have very poor heat conduction properties so even the outer skin bonds may not see the full extent of the surface temperature.

    But aluminum structures will definitely see the effect so any material selected for aluminum will need to have a high degree of temperature resistance.

    Many of the excellent candidate epoxies I reviewed will see degradations in bond strength of over 25% even at 50 deg. Centigrade (122 deg. F). For our applications this is unacceptable. As such, our criteria was that the resin must be able to retain more than 75% of its bond strength in temperatures over about 80 deg. C (176 deg. F).

    This subject is very critical to bond line design, and especially when applied to aluminum. Given the reduction in strength, the bonds in an aluminum airplane must be designed to the elevated temperature case, not the room temperature application. As such, the basis for said shear strength then becomes the elevated temperature value (about 1,000 psi or less), rather than the really attractive room temperature case, which can exceed 3,500 psi.

    Just food for thought.
     
  6. Mar 19, 2009 #6

    GESchwarz

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    When I get a chance I'll post some photos of test coupons for your viewing pleasure.
     
  7. Mar 19, 2009 #7

    K-Rigg

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    hasn't GM been using an adhesive to attach the door hinges to there cars for years? does anyone know what that adhesive is?
     
  8. Mar 19, 2009 #8

    orion

    orion

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    Actually, corrosion may not be as much an issue as would the degradation of the base material itself. But this is usually easy to evaluate since bonding agents should have, as part of their technical specifications, tables that list material degradation as a function of environmental exposure. This will include everything from simple humidity to fuels so a quick glance at the technical data sheet should tell you whether this could be a long term issue or not.

    And one other thing I'd like to pass along - this came cross my desk about a week ago and I haven't actually researched its true effect but I thought it might be worth mention. The item pointed out that one of the features of Methacrylate materials that makes them work so well for bonding aluminum is their ability to essentially self-etch the area of contact. This ability is a result of the fact that Methacrylates are actually an acid so the etching action is similar to that we see with the more conventional processes that prepare aluminum for adhesive application. The note then asked two questions: The first stems from the longer term exposure of the aluminum to the Methacrylate - in other words, during normal etching operations the acid solution is neutralized and washed off; with the use of Methacrylate it is not so it may be logical to ask whether there are any long term effects of this bonding agent being in contact with the aluminum, or does the acid get somehow neutralized as the adhesive cures.

    The second question then asked whether this condition may become a stress concentration, which may have to be considered in the design of the joint. The etching action that occurs as a function of the bond modifies the surface. This then creates a boundary between the bond affected surface and the surrounding unaffected material. Normally, surface quality changes can be stress concentrations and thus the boundary between the regions can be the initiators of crack formation. As far as I know no-one has as of yet investigated whether this is an issue or not so maybe a bit of conservativeness may be prudent in the design of said structure.
     
  9. Mar 19, 2009 #9

    orion

    orion

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    Some time back I was told that that is a Methacrylate, modified for application to steel however, I have not seen any published verification of that.
     
  10. Mar 19, 2009 #10

    GESchwarz

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    Excellent point Orion.

    Now that I've narrowed the field to tough adhesives, I need to test those that can break pairs of -4 rivets in lap shear at elevated temp. As far as peel strength goes, I will be doing my best to not design joints that subject the material to peel. Sure, there is some peel component to lap shear designs, but that can be minimized. There is nothing wrong with a "cheater" rivet in key places.
     
  11. Mar 19, 2009 #11

    Mac790

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    The Lotus Elise has bonded aluminium chassis
    full site here Lotus: Aluminium Extrusions and Adhesives

    Seb
     
  12. Mar 20, 2009 #12

    MalcolmW

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    Hello, Gary;

    you are to be commended for detailed discussion of your extensive efforts investigating a suitable adhesive for your proposed usage.

    I'm a little surprised at the results, however, I do note that you rinsed with MEK and used phosphoric acid to 'etch' the aluminum as a preparation. In addition, you were disappointed in the strength of epoxies due to failure in peel mode.

    My experience with adhesives (professional - testing adhesives for military aircraft assembly) was somewhat different than the results you reported, as was the surface preparation method (alkaline surfactant wash, followed by hot 'cleaning solution' - sulfuric acid + sodium dichromate). Using an epoxy-novolac (phenolic resin modified epoxy), produced shear strengths in excess of 3,500 psi at 200 deg. C, plus acceptable peel (I cannot remember exact numbers, however, using 0.016 aluminum resulted in metal failure).

    Nevertheless, I do encourage you to use what you have tested and found acceptable because you have established a procedure with which you are comfortable and have first-hand knowledge as to the results.

    It is my opinion that aircraft can be assembled safely with adhesives (epoxy, acrylic and polysulfide) at a slightly lower weight than mechanical fasteners. In addition, they will have greater resistance to metal fatigue due to stress distribution not being concentrated at points in the joint.

    Lastly, I believe that concerns about moisture entering the joint and causing failure are not significant, for most adhesives are well-protected by from excess weathering by using an over-lapping joint design. For greater protection from this issue, consider 'passivating' the aluminum surface with an alodine treatment (this will stabilize the aluminum by creating a metal oxide complex which does not hydrolyze readily).

    Edit note: Aluminum forms stable oxides - aluminum corrodes and forms aluminum hydroxide in the presence of water + active ion. This produces a volume change which breaks the adhesive bond, thus delaminating the joint. Consequently, alkaline cleaners should be avoided on aluminum aircraft, especially those with adhesive assembly.

    Again, congratulations on the fine work that you have done & my best wishes for your every success in the construction of your aircraft. I believe that you are on the right track.

    All the best & fly safe,
    MalcolmW
     
    Last edited: Mar 20, 2009
  13. Mar 20, 2009 #13

    MalcolmW

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    Gary;

    I forgot to mention, but there are other classes of aluminum adhesives. In particular, the adhesive / caulk that is used in gutter sealing. I believe that this is a butyl rubber material which has superior weather resistance, plus has high peel strength. It is also widely available at most hardware & discount building supply houses at a cost far below most industrial adhesives... plus it comes in a wide range of colors.

    I've never tested it, however, I have tried to tear apart old aluminum gutters and found that to be a difficult task. If it can hold up in a gutter, I suspect that it might have an application or two in holding an aircraft together, especially if you intend to use a 'cheater' rivet or two.

    Just a thought you might enjoy.

    All the best & fly safe,
    MalcolmW
     
  14. Mar 20, 2009 #14

    GESchwarz

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    Here's the pics I promised. Two shows a typical group of sheared AN -4 rivets. If you have any questions about the pics, let me know. The big group shot is of my earlier tests. I had done quite a few hybrid joints of Proseal and epoxy, which do well but of course are not nearlyu as strong as the methacrylates, but do much better than epoxy in peel and vibration tolerance. Joint 53 is noteworthy because it was actually a bad joint, in that it had come apart and re assembled while still fluid and so it had voids, as you can see in the lower right photo. Despite the defect, it held really well, breaking the three rivets shown in the upper right photo.

    The yellowish pic shows some of the peel test coupons. You can see that most of them survived the .023" 2024 T3 that was torn by the two rivets. Each joint that failed was subsequently riveted back together to continue pulling until all but the last had been pulled to failure. They all had been riveted back together with 3 rivets, because 2 would not hold, rather they sheared readily.

    The peel tests were actuated by hand with the aid of two pair of pliers. The lap shear tests were performed with a hydraulic jack and an assembly of chain, clamps, a beam and two pair of grippers, one of which is shown below.

    As Orion had suggested, heat may be a concern as these coupons were not heated during failure test. However the manufactures do certify strength well above the 150 degrees or so that the aluminum may get up to sitting outside at Mojave in the summertime. One answer to this and other concerns is to simply design conservatively by maximizing joint surface areas.
     

    Attached Files:

    Last edited: Mar 20, 2009
  15. Mar 20, 2009 #15

    GESchwarz

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    Thanks Malcolm. Regarding alkaline cleaners...For this corrosive action to take place, does it not first have to penetrate or permeate the joint? If water or cleaners are not allowed to stand at these joints, what is the risk of this corrosion actually occuring?
     
  16. Mar 21, 2009 #16

    MalcolmW

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    Hello, Gary;

    typically adhesives are used in a lap joint configuration, which provides protection from most environmental stresses. However, corrosion occurs on a molecular level, which means that atmospheric moisture / humidity can play a role in corrosion.

    Therefore, using a belt & suspenders approach, good joint design plus substrate passivation is a sound strategy for joint longevity. In addition, paint (polyurethane types) provide excellent top coat protection against corrosion.

    If you live in a low humidity area (such as southern California), and you dry your plane after washing, you are unlikely to run much risk of corrosion (within the joint) taking place. However, the future is unknown, unusual weather can occur, hangers can develop leaks and you may eventually sell your plane.

    Alodine isn't expensive (if you buy it right) and provides peace of mind.

    It's good to hear of your conscientious efforts to build your plane strongly and safely.

    All the best & fly safe,
    MalcolmW
     
  17. Mar 21, 2009 #17

    GESchwarz

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    I'll see what effect alodine has on adhesion and report back.

    I never did explain how I tested for vibration tolerance. This was really a freebie... Each time a joint was destroyed, I reattached it back together with 2 or 3 rivets. The rivets were set between an anvil and a hammer. During the setting of the rivets, some joints simply fell apart while some others failed in the subsequent pull test. The tough, elastic methacrylates survived all the riveting going on around them, whereas the brittle adhesives threw in the towel. Be advised that some methacrylates are brittle. Here again, you want adhesives that have sufficient Elongation. 50% elongation proved to do very well, as I described at the begining of this post.

    It is important to note that some of these joints that readily fell apart due to vibration, had performed quite well up to that point in lap shear.

    The riveting semed to only affect the joints immediately adjacent to where the riviting was taking place. The nature of the geometry and dimensions of the test samples seemed to isolate the severity of vibration to a very localized area.
     
  18. Mar 22, 2009 #18

    MalcolmW

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    Hello, Gary;

    here's some info on alodining (also spelled alodyning). Alodyne solution is available from Aircraft Spruce, however, the most economic source is Iridite 14-2, from which the solution is made. In particular, Iridite 14-2 is commonly refered to as "Alodyne," & is made by MacDermid Corp. There are vendors that re-sell it in small quantities (10 lbs – which will make enough solution to corrosion proof an entire small aircraft).

    After mixing the Iridite powder with water, you get that familiar looking gold/red/brown liguid. The Material Safety Sheet says it contains Chromic Acid, which explains the term "chromic treatment." Material Safety Data available at: IRIDITE 14-2

    At any rate, you won't get that "factory" look unless you submerge the parts completely. Brushing, rolling, sponging, spraying, etc. will leave it looking streaked or uneven, but this is only cosmetic, and the part is still protected. A plastic rain gutter idea for long tubing works. If careful, a sturdy wood box lined with polyethylene sheeting (must hold liquid) can be used to treat larger pieces.

    Alodyne will not produce an even finish if there are oils or unetched areas on the metal (and adhesives won't bond properly). The process I use is to wipe with MEK first to get print and oils off, then wash with ‘Dawn’ dishwashing liquid (or Alconox - an industrial detergent) and rinse. It is clean when water runs off the aluminum in sheets. Then dip (immerse) in the Iridite solution until it has a golden-brown look (about 20 minutes, sometimes more). Temperature should be above 70F. Rinse and let air dry. Paint or adhesive bond within twenty-four hours for best results.

    Even more info on ‘alodyning’ aluminum for aircraft applications: Alodining

    If you plan to 'test' alodine - maybe a gallon jug from Aircraft Spruce would be a prudent / economic step.

    Good luck.

    All the best & fly safe,

    MalcolmW
     
    Last edited: Mar 22, 2009
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  19. Mar 23, 2009 #19

    GESchwarz

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    In my aerospace career I have seen iridated finishes, usually hand touchup work. It appeared as though this process left something on the surface that is sort of a brownish green in color. I can't imagine methacrylate having a good bond to the aluminum if there is this material in between. The iridite touch up process that they always used was to apply with a brush and let dry. but you were suggesting that you leave it on wet for 20 minutes or so then rinse. What if it was allowed to dry in that 20 minute period and then an attempt was made at a rinse...would it wash off so that I could bond, or what? My experience with the phosophoric acid and distilled water rinse was not good...bonds to virgin, freshly sanded aluminum consistantly out performed the phosphoric acid treated surfaces.

    We used to etch metal welds with hydrochloric acid to be able to visulize the welds and as prep for dye penetrant inspection. The acid opened up the crystiline structure of the metal. It would seem that this kind of treatment would be good for adhesive bonding.

    My thinking is that what the sanding process does to the aluminum mechanically, is essentially the same thing that acid does chemically, and that is to expose un-oxidized aluminum. Am I right or wrong? I understand that these acids corrode or otherwise create an aluminum oxide film that prevents further corrosion. That's all well and good for protecting the aluminum, but it seems to be counter productive to what were trying to do here, which is to get a good adhesive bond.

    What can you tell me about the iridite residue? Or should there never be a residue, and how can that be 100% prevented?

    I'm confused. :confused:
     
    Last edited: Mar 23, 2009
  20. Mar 23, 2009 #20

    MalcolmW

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    Hello, Gary;

    First things first: It is NOT necessary to ‘buff’ or abrade the surface of aluminum to obtain a high-strength adhesive bond. Forget the Scotchbrite scrubbing pads. Roughing up the surface can increase the surface area to which the adhesive can attach, but this isn't necessary and can 'embed' contaminants that may cause problems later.

    Adhesive bonding is a chemical bond, not mechanical (Van der Waals forces).

    Consequently, the best adhesive bonds are those that have the greatest 'chemical compatibility.' In the case of aluminum which comes from the mill, it normally has a thin (molecular) surface layer of oxide contaminated with oils, etc. This layer of oxide can hydrolyze into a hydroxide. However, (bear with me), aluminum has some odd chemical properties because it is amphoteric (can be attacked by both acid and base), which makes it vulnerable to corrosion.

    So, you, the aircraft builder, want the aluminum to bond well and resist corrosion.

    The aircraft industry has found (lots of experience, particularly for military aircraft) that a chromate conversion (properly done) produces a corrosion resistant surface to which adhesives bond well. The 'chromate' layer, a molecular layer of chromate, has a golden-brown color. The 'green' surfaces, I believe, are surfaces which have had a primer sprayed on.

    Alodine or iridite is used to 'CONVERT' the aluminum surface, or give it a molecular layer of chromate. To do this properly, the alodine or iridite solution must have enough time to react chemically, then the surface MUST be RINSED clean, so that there are NO evaporated salts left on the surface.

    Think of it this way: If it isn't rinsed, you have a surface for bonding upon which there is a layer of salt. Obviously, this will get between the aluminum and the adhesive and will interfere with the bonding process. In the adhesives business, 'cleanliness in next to godliness.'

    I hope that this hasn't been too pedantic. I do want you to be successful in your efforts, and I offer these comments based upon professional experience.

    All the best & fly safe,

    MalcolmW
     
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