epoxy two part varnish before dope

[idamon]

Should you cover wood surfaces that will be doped for fabric covering or should you leave them naked like glue application locations? I am using the randolph two part epoxy varnish.
Thanks

[Neil]

By all means, varnish your wood. Also any areas that create a sealed compartment should have a small vent hole to balance atmospheric (altitude) changes. Enclosed areas should be internally varnished before closing.

[idamon]

Are there any special considerations that must be taken into account when picking fabric or dope when covering over the epoxy varnish?

Thanks again

[Neil]

The best advice is probably to choose a process and follow their manual. The Poly fiber process suggest a first coat of 2 part epoxy varnish, thinned for penetration, followed by a second un-thinned coat. At least that is what I remember. You must seal the wood if you want it to last. You should coat the inside of any bolt holes that pass through the wood as well. Thinned 2 part epoxy applied with a pipe cleaner is the method I used in the holes. Moisture in the wood will attack the bolts over time if the holes are not varnished.

[idamon]

excellent points. How do you prepare the bolts to prevent corrosion? Mine seem to have a goldish colored coating that I assume is cadmium, should I clean, prime, etch, and paint them still? What kind of primer and paint would you recommend? What about metal parts that move and rub against each other like hinges, should they be painted even though it will rub off in areas quickly? Thanks

[Neil]

If you varnish the holes in the wood there is no need to provide any other coatings to the bolts. The varnish seals the wood pores keeping the moisture from the bolts. All the steel parts should be media blasted (nothing too harsh) and primed with epoxy primer. The aluminum parts can be left bare but Alodine coating is a good thing to do. This involves a chemical cleaning/coating which you can do yourself at home.

[highflyer]

I would be extremely reluctant to put anything on the wood surface where I am going to bond fabric. I still recommend cotton fabric over wood. I keep remembering that Steve Wittman, whose expertise I looked up to for years concerning wood and fabric airplanes. He was killed when the fabric came unbonded on his wing. I think I want whatever soaks into the fabric to also soak into the wood and make a really good bond. I can remember using cotton fabric soaked in really thin weldwood glue and rolled onto the wing plywood. Generally we bonded the fabric to the wood with a liberal application of NITRATE dope. Butyrate doesn't give a sufficient bond. Steve proved that the hard way.

[idamon]

Great, exactly what I didn't need... two completely different opinions. Which is correct guys? I guess I need to buy a fabric system and read their manual?
Thanks

[Tom Downey]

Stewarts are on Utube.

MY opinion is when you have ribs to stitch to you should, when you cover wood sheet, use a good fabric cement, like the water based stuff Stewarts use or the UA 55 from Airtech.

Before I covered my Fairchild 24 I did some test. I cut a 6" wide strip of birch ply and started with bare wood for all three cements UA 55 from Airtech, Polyfiber cement, and the blue glue from Stewarts, then I Varnished the ply with spar varnish, and attached three more strips of 102 ceconite, then I varnish the ply with a urethane two part varnish from Smith and company, and attached three more strips of ceconite.
I then allowed the test strips to dry over night, and pulled each strip from the ply with a fish scale, all failed at the same weight, and every time the wood was what failed.

[WLIU]

If it helps, I will offer the info that every modern recovering method uses polyester fabric and has you prime tubing and varnish wood before adding fabric. I pull 6G's two or three times a week flying a Pitts covered with Cooper Superflite. Fabric over varnished wood and primed tubing. No problems with adhesion.

Modern fabric adhesives work fine over varnish. But as mentioned previously, you need to follow the directions and not fudge as you go along.

Today I would not own an airplane covered with cotton. Grade-A is very hard to find these days and it does not last like the new materials. And it degrades by the calendar. Why guarantee that you have to rip it all off and do it over any sooner than you have to?

Every manufacturer publishes manuals with enough detail that you can do a decent job. Do be careful of shelf life though. Some of the new water based finishes can only sit on your shelf for a few months before they have to be thrown away. A friend learned this by having to stip down and redo his wings when the finish dod not come out right. So put off buying finishing stuff until you really need it.

Best of luck,

Wes


P.S.

I visited Steve Whitman at his home at Leward Air Ranch a few months before his crash. Very accomplished guy, but very willing to answer questions about his latest project all laid out in his hangar.