Prepping plywood for T-88.....

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I remember reading that plywood needs prepped before being glued with T-88 but I can't find that discussion again.
If I recall correctly.....the ply is prepped by scraping with razor blades or sanding to get the coating off.
Can someone expand on this....???

Kevin

lakeracer69

Well-Known Member
I just use acetone to clean the surface before epoxying. It depends what you are trying to adhere to whatever. Planing is always a better way to get a clean surface. I remember an EAA book saying never sand surfaces before gluing , on spars, etc. Kind of makes sense. Sanding leaves a bunch of burrs microscopically.

BBerson

Light Plane Philosopher
HBA Supporter
Plywood has a compressed shiny surface. AC 43.13 paragraph 1-6 advises to "very slight sand plywood with 220 grit" , " greatly improves penetration".

Rockiedog2

Well-Known Member
HBA Supporter
Birch ply is harder to get penetration than mahogany. I prefer mahogany myself. Sometimes there is a waxy coating from the manufacturing process. Get the light right and you can see it. That has to be removed and in spite recs not to sand that’s what I have always done. With like 80 grit and a vibrator sander and remove the dust with compressed air and a stiff nylon brush. If you don’t remove it you may have unairworthy parts.

wsimpso1

Super Moderator
Staff member
Log Member
The comment above recommending that sanding NOT be performed surprised me, as it is counter to my training and background.

I checked the websites of a number of epoxy suppliers, and without exception on the sites I visited, they all reccommend sanding wood surfaces and removing all dust before bonding. In particular, West Systems, System Three, T88, Smoothon, and Wessex Resins all recommend sanding of wood surfaces prior to bonding.

• Apply neat resin to wood surfaces;
• Let the applied resin stand for a few minutes and reapply where the resin had left a dry looking surface;
• Modestly fill epoxy to make it fill gaps better, then apply it to the already wetted surfaces for assembly and clamping;
• Removing oils by performing a solvent wipe was also recommended.
Billski

BBerson

Light Plane Philosopher
HBA Supporter
I think sanding is preferred when planing is impossible, such as cross grain or scarfing.
AC 43.13 has changed over the years. My 1988 copy says plastic resin is the most common glue. New AC does not recomend plastic resin. It is advisory, not all agree.

Tam Dl

New Member
I don't have the aircraft skinny on this but two points:

1) Used to clean a lot of stuff with acetone, but would have occasional problems, so I would use acetone first then alcohol. Well I found out why I had the problems, there is oil in acetone, it was on the MSDS I think someone pointed that out, whether it is there for a reason, or it is a contaminant, there it was in a trace. There is medical grade stuff, but I have no experience with it. So I keep that in mind and only use acetone if it is necessary to cut through something else, and the surface is non-absorbent.

(Speaking of acetone, there was a time when I cleaned epoxy from my skin with it. If I had a little residue. Turns out fat and soap work better (never use acetone). If I get a little sticky on my hands because the gloves broke or whatever, I rub in a small amount of butter or marg. , then I wash off with generous licquid hand soap. The local dollar store sells large bottles of Spa Soap that wash epoxy off directly, which is a little frightening. Anyway, if you look up Gougeon Soap online, you can probably find the full recipee. Mostly I just need spot cleaning so I don't any longer making up a full recipee, though it keep well at room temp.)

2) Epoxies don't really penetrate. Not sure what the correct term is, you do want them to "wet" maybe. But as to their sinking deep into the wood, it does not happen. The molecules are too large, or something. I have no direct experience with penetrating epoxies like rot doctor, but anything as wetting as WEST, etc... and certainly T88 is basically on the surface. I did a deep dive on this when I had some ply with a very light surface veneer about .5 mm, and the glue line was iffy (not an aircraft project). I tried everything I could find at the time to get the epoxy to get in there, and it never did. There may be better approaches these days, but that was my experience.

Tam Dl

New Member
If you blow with compresed air be careful that you don't blow on oil. I have two oiless compressors, but am still low mileage on them, they would be better. If one does a lot of nail gun work, oil can get back into the hoses.

BJC

BJC

Well-Known Member
HBA Supporter
Good points, TD, and welcome to HBA.

It’s prudent to test each new can of acetone for contaminants. I have learned to pour some into a clean container, swish it around, and let it evaporate. If there is any residue, I won’t use for any use in involving bonding.

BJC

Yellowhammer

Well-Known Member
Bottom line is this, sand the plywood for two reasons. One, to remove any non-desired film, and second, to scuff the surface for better adhesion. Something for the epoxy to sink it's teeth into.

I always wipe the plywood off with a clean, damp, towel before epoxy treatment.

Hope this helps.

Yellowhammer

wsimpso1

Super Moderator
Staff member
Log Member
Since there is so much talk about this, I suppose we should get into what is happening and how it can go wrong...

Wood is made up of hollow cellulose fibers, glued together with lignin, and still contains what is left of sap, which includes other oily or resinous materials.

Cutting open these cellulose tubes at the surface allows the glues (whichever ones you use) to flow into the opened fibers at the surface and greatly increases the surface area that the epoxy can grab. Machining, sanding and even scraping with a cabinet scraper does this when the cutting tool is sharp, and they all have been demonstrated to work well.

An easily imagined error state is a planer with cutting edges that are becoming dull. While sawdust is created, the last few thousandths of the wood is not removed, but is instead burnished, with most of the surface fibers compressed and not open to our glue. We can make it worse by running slowly or dwelling in spots, which melts and flows the lignin, sealing over the wood surface with a now continuous layer of this resin. My wife (a material scientist with a long background in polymers) describes a burnished lignin surface a "low energy" surface and thus likely to produce only a poor bond.

Robust prevention may be tough to achieve by keeping cutting tools sharp, but sanding, as recommended by the makers of our common epoxies, is pretty darned robust at making sure the surface is all opened... and the wife describes a sanded surface as a "high energy" surface and thus more likely to produce high strength bonds.

Next, epoxy penetration into the wood. Gougeon Brothers (makers of the excellent West System and ProSet epoxies) has done extensive testing on their epoxies and on "penetrating" epoxies. Their finding is that "penetrating" epoxies are poor wood adhesives. These products are epoxy thinned with a solvent. and in all manner of tests, they produce lower strength bond than with the resin systems we know to work well. The goal in bonding wood is to produce a bond where wood fails and no failure occurs in the cured adhesive. These "penetrating" epoxies frequently produce failure in the resin... So, do not worry over the lack of penetration due to the formulation of the common resins. West and T88 are the standby products because they have been demonstrated to be reliable, durable, and a pleasure to work with.

Now to acetone. It is a nice solvent if it does not have oil in it, as observed by other folks. I attended a session on repair of composites some time back, and the trainer recommended wiping a sample of your acetone on a clean glass surface and watching it evaporate. If it does not leave the glass as clean as it started, it has oil remaining from the distillation process. This oil will interfere with bonding. The only acetone his aerospace repair facility allows on site is reagent grade acetone. Yeah, it costs more to distill the acetone to that level. You know where the impure condensate from distilling goes? It becomes hardware store acetone. Alternatives are other solvents, which should also be checked by the same methods, but are way more likely to leave really clean surfaces.

Water on wood anytime near bonding time? Not if you want good bonds. Every bit of instruction on bonding with epoxy, painting with various systems, etc say to dry a water wetted surface and then allow it to stand for some period before the next operation. Yeah, even water borne paints (Stewart System) tell us to let them dry for intervals after wetting with 90% propanol wipe. Wood stores water inside the fibers and then gives it up slowly over time, so I would avoid water application anytime near when I expect to bond surfaces.

Billski

David L. Downey

Well-Known Member
At my current employer (for 3 last days yet!) we had a major issue with acetone purity. Our specs only required "technical" grade for pre-bond or pre-paint cleaning. However our specs did not address container purity or contamination... we learned that our acceptably pure technical grade was being shippen in tank cars and drums that were reclaimed and there was not control over the prior contents! we ended up changing the specs to require "reagent" grade - a much more costly grade, but a grade that comes from the manufacturer in known clean vessels. Subsequent to that issue we had in-house repackaging to smaller bottles - that turned out to be insufficiently solvent resistant and they woudl very slowly breakdown on the inner surfaces, yet again contyaminating the solvent. Today only reagent grade, in the manufacturer package acetone is allowed for pre-bond and pre-paint cleaning!

Rockiedog2

Well-Known Member
HBA Supporter
The comment above recommending that sanding NOT be performed surprised me, as it is counter to my training and background.

I checked the websites of a number of epoxy suppliers, and without exception on the sites I visited, they all reccommend sanding wood surfaces and removing all dust before bonding. In particular, West Systems, System Three, T88, Smoothon, and Wessex Resins all recommend sanding of wood surfaces prior to bonding.

• Apply neat resin to wood surfaces;
• Let the applied resin stand for a few minutes and reapply where the resin had left a dry looking surface;
• Modestly fill epoxy to make it fill gaps better, then apply it to the already wetted surfaces for assembly and clamping;
• Removing oils by performing a solvent wipe was also recommended.
Billski
Bill
My copy of 43.13 is the 1965 version when Weldwood Plastic resin was what the pros around here were using (so that's what I used). It was one of the recommended glues back then along with resorcinol. Anyway, Ch 1 Sec1 para 5 says sandpaper must never be used to smooth sortwood surfaces to be glued. That's where that came from.

Rockiedog2

Well-Known Member
HBA Supporter
Since there is so much talk about this, I suppose we should get into what is happening and how it can go wrong...

Wood is made up of hollow cellulose fibers, glued together with lignin, and still contains what is left of sap, which includes other oily or resinous materials.

Cutting open these cellulose tubes at the surface allows the glues (whichever ones you use) to flow into the opened fibers at the surface and greatly increases the surface area that the epoxy can grab. Machining, sanding and even scraping with a cabinet scraper does this when the cutting tool is sharp, and they all have been demonstrated to work well.

An easily imagined error state is a planer with cutting edges that are becoming dull. While sawdust is created, the last few thousandths of the wood is not removed, but is instead burnished, with most of the surface fibers compressed and not open to our glue. We can make it worse by running slowly or dwelling in spots, which melts and flows the lignin, sealing over the wood surface with a now continuous layer of this resin. My wife (a material scientist with a long background in polymers) describes a burnished lignin surface a "low energy" surface and thus likely to produce only a poor bond.

Robust prevention may be tough to achieve by keeping cutting tools sharp, but sanding, as recommended by the makers of our common epoxies, is pretty darned robust at making sure the surface is all opened... and the wife describes a sanded surface as a "high energy" surface and thus more likely to produce high strength bonds.

Next, epoxy penetration into the wood. Gougeon Brothers (makers of the excellent West System and ProSet epoxies) has done extensive testing on their epoxies and on "penetrating" epoxies. Their finding is that "penetrating" epoxies are poor wood adhesives. These products are epoxy thinned with a solvent. and in all manner of tests, they produce lower strength bond than with the resin systems we know to work well. The goal in bonding wood is to produce a bond where wood fails and no failure occurs in the cured adhesive. These "penetrating" epoxies frequently produce failure in the resin... So, do not worry over the lack of penetration due to the formulation of the common resins. West and T88 are the standby products because they have been demonstrated to be reliable, durable, and a pleasure to work with.

Now to acetone. It is a nice solvent if it does not have oil in it, as observed by other folks. I attended a session on repair of composites some time back, and the trainer recommended wiping a sample of your acetone on a clean glass surface and watching it evaporate. If it does not leave the glass as clean as it started, it has oil remaining from the distillation process. This oil will interfere with bonding. The only acetone his aerospace repair facility allows on site is reagent grade acetone. Yeah, it costs more to distill the acetone to that level. You know where the impure condensate from distilling goes? It becomes hardware store acetone. Alternatives are other solvents, which should also be checked by the same methods, but are way more likely to leave really clean surfaces.

Water on wood anytime near bonding time? Not if you want good bonds. Every bit of instruction on bonding with epoxy, painting with various systems, etc say to dry a water wetted surface and then allow it to stand for some period before the next operation. Yeah, even water borne paints (Stewart System) tell us to let them dry for intervals after wetting with 90% propanol wipe. Wood stores water inside the fibers and then gives it up slowly over time, so I would avoid water application anytime near when I expect to bond surfaces.

Billski

I liked the explanation about penetration. I think in my case anyway that term was carried over from the plastic resin days when 43.13 recommended 125-150 psi gluing pressure(1965 ver Ch1 Sec1 para10). The logical conclusion to force the glue into the wood. And the IA who taught me was big on high pressure for "penetration".
And since we(I) don't use near that much pressure with T88 it follows that we're not trying to force the glue into the wood and so yeah "penetration" isn't the proper term. Ok good we got that. I guess.
I like to clamp til get squeezeout all round and let it sit for maybe 10 minutes or so and come back and apply just a little more pressure then let it alone. I don't like spring clamps or weights cause I always thought that would squeeze too much glue out and risk a dry joint.
I proved T88 isn't foolproof when I had an inflight structural failure in an Acrosport. Drag wire pull block glue joint sheared(dry joint); I believe Billski mentioned the possibility of sheared joints in an earlier post. Well, that's sure what happened to mine; no wood tear just a fairly clean shear break in the glue itself. That joint was glued vertical and too much glue squeezed out and I missed it. I glue most everything horizontal anymore. In hot wx T88 stays pretty thin for a while.
good info Billski. And others. Glue threads stir up almost as much interest as oil threads but w/o so much passion. Some guys get so worked up they wanta fight. I like it when that happens. It's hard to pick a fight around here. Everybody's so polite. Well, just a random thought there.

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Pops

Well-Known Member
HBA Supporter
Log Member
I'm ready Joe, Losing a fight with my wife has too many consequences

I agree with Joe. I built a KR-2 back in the 1970's and used Weldwood. Did the same , I would clamp the wood until a little glue runout around the edges and then later add a little more pressure. Using spring pressure, you don't have control of the pressure, easy to get to much and squeeze most of the glue out for a starved joint. Tried to do all of the gluing horizontal if possible to stop any runout and starved joints. I made wood glue samples with every glue batch. Tested the samples in a vice to destruction. The wood failed instead of the glue joint in all of the test. Also saved samples for the FAA. Things were a lot different in the FAA inspection at that time. The inspector would want glue samples to check for his self. Also every thing had to be inspected before closing up. ( Box spars , etc ).
I love using T-88, so much easier to used to get a good joint.

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wsimpso1

Super Moderator
Staff member
Log Member
Bill
My copy of 43.13 is the 1965 version when Weldwood Plastic resin was what the pros around here were using (so that's what I used). It was one of the recommended glues back then along with resorcinol. Anyway, Ch 1 Sec1 para 5 says sandpaper must never be used to smooth sortwood surfaces to be glued. That's where that came from.
Oh, cool. Now we know.

My copy of 43.13 dated 2001 says similar stuff about sanding. Interestingly, there is no mention of epoxy in 43.13 chapter 1, and the good book also says to adhere to the adhesive manufacturer's recommendations, then goes on in Chapter 1.6a to mention using 220 grit sandpaper to assist bonding of plywood. Imagine that, a government document giving us conflicting information. LOL.

Is anyone still using Weldwood Plastic Resin? Looks like economical stuff, but it requires high clamping pressure and planed or machined surfaces to work well. I think I will stay with my West System...

Billski

BJC

Well-Known Member
HBA Supporter
Is anyone still using Weldwood Plastic Resin?
I know one commercial wooden propeller manufacturer who is.

BJC

don january

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HBA Supporter
Log Member
Awhile back my oh man made a scale for mixing Weld wood resin water mix and it was a beam scale. The glue was great and we put a set of rotors for a Gyro cop together with it. Took the Gyro out to tow behind the car and it shook so hard the stick broke off in my hands but the glue held upThank goodness the craft was still on the ground. A bit out of balance I'd say. I have found that the two pieces of wood your gluing together should fit tighter then duck puey on the flats and have either drill hole starts or valley grooves for room for the glue when clamped. My Taylor-monoplane Fus is fastened together with weldwood and I have had to remove a few over drips and some I waited to long and the wood separated before the glue popped.

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TFF

Well-Known Member
There are some old timers that will and there are a few who wish it could loose its stigma because the known issues are easily solved. Essentially don’t be shy of glue, lots of traditional clamping pressure, and build in a no AC garage in Florida during the summer.

Temperature seems to be a big tripping point. Another site build thread had the builder complaining that T-88 said can glue at 58F or something and he had the shop at 60F and the glue was not getting hard. Of course no one is heating when they are not in the shop too. Finally people convinced him to build a heat box and then he got it. That is kind of what happened to Weldwood. People building in cold garages treating it like epoxy, just slathering it on and pushing it into place. It will hold for a couple of years, and then it will pop apart.