Let's say I can only build one fuselage side of my plane and it sits for a year...should I varnish it before I store it and then sand off the varnish in the areas to be glued when I'm ready to assemble the other sides of the fuselage????
Let's say I can only build one fuselage side of my plane and it sits for a year...should I varnish it before I store it and then sand off the varnish in the areas to be glued when I'm ready to assemble the other sides of the fuselage????
First, if unfinished wood is left to sit for a year, it will be affected by changes in humidity and it will oxidize.
The next question is what glue you are using? If you are using an epoxy, I suggest wetting the surface to be glued with epoxy now and then sanding it with 80 grit just before doing the glue up next year. The West System manual talks about gluing to a fully cured surface.
Varnish the surfaces that will not be glue joints.
Best of luck,
Wes
Wood swells and shrinks with changes in humidity but lumber cut with the grain vertical, like that used in aircraft building resists warping and twisting while that is going on. Keep it out of direct sunlight and all will be okay. The wood may turn a darker color due to oxidation but it's going to do that even when varnished. FWIW, some of the wood at a popular aircraft wood supplier has been sitting in inventory well over 1 yr....unvarnished.Quote:
Originally Posted by jtrom
So if I use T-88, I can glue to a surface of cured epoxy varnish without losing strength?Quote:
Originally Posted by WLIU
Not a chemical engineer here but, as I understand it, epoxy is very much a generic term. The working characteristics of hardware store, 5-minute epoxy are significantly different from structural epoxies like T-88. Where your epoxy varnish falls in the spectrum is an unknown. I think its job, after all, is just to seal out moisture etc..
I have glued to paint in the past and had the joint fail between the original paint coating and the wood on which it was applied. The new glue bond to the old paint was just fine, it was the adhesion of the paint to the original wood that was not strong enough. In short, it pulled the paint right off. That having been said, how much varnish you have to sand off or chemically remove to achieve an aircraft-quality joint is something I have not heard. Maybe someone has tested different surface treatments?
Also, if you have the luxury of being able to control the humidity in your shop/hangar, keep it constant, stack your wood so that there is no bending stress and your wood will behave for a long time.
Best of luck
To answer the question - If you have a surface that was properly coated with an aircraft grade epoxy like T-88, you should be able to come back later, abrade that surface and get a full strength glue joint. I work with West System but the T-88 documentation shiould say the same thing. Scratch the old surface up with 60 or 80 grit sandpaper until the entire surface has been scratched up and the new epoxy can "bite" into the scratches.
As with most types of gluing operations, you should try this on scrap to demonstrate that you will get what you want. Take one piece of material and apply your epoxy. Allow to cure and sit for a few days. Abrade and attach a second piece of scrap. Allow the new epoxy to fully cure. Break the sample and observe where it fails.
Best of luck,
Wes
N78PS
System Three says essentially the same thing....surface prep for cured epoxy requires scuff sanding to get a dull finish then glue away.Quote:
Originally Posted by WLIU