Prepping plywood for T-88.....

[Pops]

I don't like to use T-88 under 70 degs and not over 90 degs. Viscosity changes to much, espectially at the higher temps where the epoxy will run out of the joint even if horizontal and level.

 

[BBerson]

Weldwood Plastic Resin doesn't need as much clamp pressure as Weldwood Recorcinol.
I never had any issue with cold as per the instructions on the container. Just don't add any more water after mixing.

 

[Rockiedog2]

IIRC T88 used to claim it could be used down to 35 degrees and be used to glue wet material. Well, I don't know anybody who would do that. But that's what it said.
I think there's still a lotta planes out there with plastic resin glued wings. But, personally, I don't see any reason to use it on aircraft structure with T88 available. When the IA who helped me along finally decided to try T88 he was amazed how much easier and more reliable it was than plastic resin. Same thing with Stits(polyfiber). Scoffed at it for years and kept using cotton and dope and then finally tried it and never went back.

 

[mcrae0104]

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.

From Gluing & Clamping: A Woodworker's Handbook:

"Sanded surfaces as a rule do not make good surfaces. In edge-to-edge work, sanding rounds edges and removes more wood from the softer areas of the board than the dense areas. Consequently, a sanded surface is less uniform than a well-planed board. Also, sanding with coarse grits creates torn fibres that are likely to pull loose under stress."

While I don't think this invalidates the material scientist's perspective, from a practical viewpoint, we're gluing the sides of the straws (cellulose) to each other and we don't want to hack them up so that we get the most bonding surface possible. The author also cautions against burnishing the wood with dull planer or jointer knives (just as Billski did).

Then again, nobody's pulling out the trusty #7 jointer plane to surface a sheet of plywood...

 

[blane.c]

Hand plane isn't normally sharpened flat but shallow curve and so planing a surface results in a series of shallow troughs going with the grain, using a joiner there will be a series of shallow troughs going the opposite direction, that is across the grain. So how is that fundamentally superior to sanding which leaves shallow troughs by removing a little extra of the softwood randomly.

I mean nothing is truly a flat surface bonded to another flat surface, hmm, maybe we laser prep?

 

[Rockiedog2]

When using T88, lots of this is trivia. It doesn’t have to be perfect. That’s why we use it. Get it good enough, glue it up, and on to the next one

 

[mcrae0104]

So how is that fundamentally superior to sanding which leaves shallow troughs by removing a little extra of the softwood randomly.

In edge gluing, you're bonding to the side of a bundle of straws. Sanding the bundle of straws gashes the sides of them open or crushes the cells. There is more on this in the Forest Product Laboratories Wood Handbook.

 

[rollerball]

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!

How much of it do you use? Can't you acquire your own clean containers which you ship back to the supplier empty each time to be refilled with technical grade? Might be worth looking at if the difference in cost between the grades is significant. Sorry about the slight thread drift.

 

[David L. Downey]

How much of it do you use? Can't you acquire your own clean containers which you ship back to the supplier empty each time to be refilled with technical grade? Might be worth looking at if the difference in cost between the grades is significant. Sorry about the slight thread drift.

I am yet workign for the capital B - and we use vast amounts of acetone. the actual problem was that the vendors did not need to ship clean because hte spec does nto requiere it and the spec custodian(s) said thye woudl not change the spec since we were buying a clean grade and no one would ship a clean grade in dirty vessels...! stupidity on both sides of that coin.
back to the thread. I suspect that the high pressure helps the adhesive wet the microsurface and comntrol the bondline to minimum gap. It is possible that those adhesives are not really happy wetting the surfaces ad we know from testing that they don't like fat bondlines. Epoxies though, in general, assuming that they wet the surfaces being bonded, often need some minimum bondline thickness to accomodate the shear lag due to the greatly different stiffness of the woods and the cured adhesive; viscosity usually helps with this...as does wetting/squeegeeing all surfaces to be bonded with the mix as soon as it is blended and then getting the assembly in order.

 

[stanislavz]

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.

In my post life, when we (me and one of my buddy) were gluing aqau-tanks, after acetone you have to wipe it with gasoline solvent. One of easiest to find supply - good quaility lighter fluid, like Zippo 3141EX or similar. It have to come from metal can and no yellow color.

 

[blane.c]

Business managers (BM's), have contributed highly to the ruining of the USA. I call them Bowel Movements (BM's)

 

[ToddK]

Uncle Tony says to give the plywood a very light sanding. That's good enough for me.

 

[Rockiedog2]

In edge gluing, you're bonding to the side of a bundle of straws. Sanding the bundle of straws gashes the sides of them open or crushes the cells. There is more on this in the Forest Product Laboratories Wood Handbook.

View attachment 104826

The more I hang around here the more I realize how little I know!
Good one McRae!

 

[BBerson]

Yeah, a bit scary reading all the details in the Wood Handbook.

 

[Rockiedog2]

Yeh there’s such a thing as too much knowledge.
I didn’t know all that stuff Mcrae posted about straws but instinctively or something like that knew not to do butt joints if possible. Think that’s what they called edge gluing

 

[chriseakin]

Boat builders use scarf joints - edged planed to matching angles - rather than lap or butt joints. Are scarf joints used in aircraft?

 

[Rockiedog2]

The center section top wing on a Pitts is a scarf joint of sorts. Only one I remember in the planes I’m familiar with

 

[mcrae0104]

Boat builders use scarf joints - edged planed to matching angles - rather than lap or butt joints. Are scarf joints used in aircraft?

Yes, absolutely. See AC 43.13.

 

[Pops]

I have built 3 all wood homebuilts and yes, scarf joints in all of the plywood.

 

[wsimpso1]

Yeah, a bit scary reading all the details in the Wood Handbook.

The wood handbook was based on adhesives that needed really good fit up, nicely planed surfaces, and serious clamp loads. We have T-88 and West, which will fill gaps, do not need much clamp load, and will give you glue joints that will hold better than the wood for longer than anyone you know will live. Be happy...

Billski