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old man emu

Resorcinol, Epoxy or Polyurethane?

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My old apprentice master taught me to use Resorcinol when constructing wooden airframe assemblies, but that was years ago, and glue technology, like everything else has progressed. Now we have the two-part epoxy glues and the straight from the glue pot polyurethanes. I'm not going near PVA glues.

 

Resorcinol is a 2-part glue, making high strength bonds, but is not gap filling.  Since it is not gap filling the contact surfaces to be glued together have to be near perfectly fitting, which means that cutting of mating parts has to be as close to perfect as possible, and the parts have to be clamped hard during curing.  Here's some info about its pros and cons: https://www.christinedemerchant.com/adhesive-glue-resorcinol.html

 

Epoxy is also a 2-part glue, making high strength bonds and is gap filling. As with Resorcinol, the two components must be mixed in the correct proportions for complete setting of the glue compound. The strength of bond formed is dependent on the formulation of the resin and hardener - probably the factor which affects the price. Since epoxies are gap-filling, the accuracy of fit of components is not as critical as with Resorcinol, however, moderate clamping of the joints is necessary to reduce the width of the gap.

 

Polyurethane is a one-part glue, making strong bonds, but a good gap filler. It is not as strong as epoxy in wood-to-wood joints, but making bonds that are stronger than the wood. Like epoxies, the formulation can be dictated by price. Apart from the need to have close-fitting joints, care must be taken to prevent foaming of the glue which occurs in damp conditions. Although the foam might appear to be filling gaps, the glue joint is weaker than where the glue has not foamed. Here's some guff 'n' stuff about polyurethanes https://www.christinedemerchant.com/adhesive-glue-polyurethane.html

 

So, what type of glue would you recommend now for airframe construction?

 

 

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Epoxy is great for amateurs like me who can't achieve the sort of precision required by resorcinol.

One downside of epoxy is heat; you can undo a cured epoxy bond by heating- whether you intend to or not.

That makes it unsuited for areas near hot engines. That's why I used mostly Vinyl ester resin in my engine cowl.

 

I believe resorcinol is pretty tricky to use, but is a "forever" job.

Edited by Old Koreelah

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Most wood butchers (like me) are using West System products.

Have also used some Bote-Cote products - didn't like the filler power that much.

I've just ordered some Epox-E-Glue from Boatcraft Pacific to try out (for models firstly, but I'll use it on the big thing too)

Mate is building his fullsize TR.9 using West System products.

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I only trust west systems.

 

Simple, easy and well known.

 

I have tried other brands and not gotten the consistent quality needed.

 

About to do mods to glass boat and will use west, ply and micro etc.

 

Also making a diy safety collar for hull. Too many old nuggets drowning lately. That is epe foam probably.,

 

Boat coat has a good name just never used it.

 

I feel the low % part 2 expoxies are inferior. Ratio gets a bit critical, west is more accommodating to diy.

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West System is fantastic for coating timber as it has a "wicking agent" in it to help penetrate the timber. It is ok for laminating fiberglass but there is better.

West also have a great range of fillers and other chemicals for different uses which are handy.

I am keeping an eye on the cyanoacrylate glues as well. They now have gap filling slower and less brittle ones.

 

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7 hours ago, Old Koreelah said:

 

One downside of epoxy is heat; you can undo a cured epoxy bond by heating- whether you intend to or not.

That makes it unsuited for areas near hot engines. 

It depends on which epoxy you use.

One type I use has to be hot cured at 96° C minimum (180°C max cure) or it won't be properly cured and is good to 120°C in normal use.

 

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I have never used Poly u, mainly because I am happy with what I have used for years.

I used Resorcinol when I built a yacht, to ply the double diagonal planking and it was really good.

I used epoxy when I built the Corby, which is 20 years ago and it is still going strong. no problems with heat. An advantage of epoxy is that you will need to seal the whole timber frame usually and it is compatible with epoxy varnish.

When I started building I went to the University in Rocky as they had done experimenting with adhesives and their recommendation was to use epoxy.

I have read a bit about poly u in woodworking magazines and it is not touted as being as good as epoxy.

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