Hatz Biplane Association

Is it OK to apply system three clear coat to the ribs before mounting them on the spar (I'm about to order 1.5 qts to start)? Then, apply T-88 where the rib contacts the spar. My plan is to clear coat all interior parts, and use the Stits/Poly EV400 on the exterior parts. I'm thinking about using system three WR-LPU Topcoat (is a two-part linear polyurethane coating) on sections exposed to the sun (instrument panel & floorboards).

You don't glue to the wing ribs, just rib stitch. Put a piece of tape the length of the top and bottom of the rib. I used 3/4" wide plastic box tape... the kind with the little reinforcing ribs in it. Don't use masking tape. Holds moisture.
And the Randolph "green stuff" only works with Randolph covering products. If you use another process, use their "pink stuff" or "blue stuff". Thin it a little and it will penetrate better.

Jeff Moore

OK, it sounds like EV-400 or "green stuff" where the fabric will attach.

Yikes, EV-400 is $146/gallon. So I probably don't want to use it to varnish my entire wing structure. Easy enough to not varnish the leading edge tube, and the trailing edge strip and to use some other for the "faces" of the spars, and ribs (which I suspect will take a lot of varnish, probably a fair amount of waste, ESPECIALLY if SPRAYED on), etc. Then I can go back and brush on EV-400 on the leading and trailing edges where the fabric will attach.

What about the surfaces of the rib cap strips that are in contact with the fabric? Are those areas glued or just rib stitched. If glued, then is it practical to sand off the "inferior" varnish from those surfaces and brush on the EV-400 there too?

Thanks for your input so far.

I used exterior grade polyurethane spar varnish on all wood that wasn't an anchor for the fabric. This varnish will lift and or turn to mush if exposed to your covering process. I believe I used two coats.
As far as raw wood... don't leave any of your wood raw. I used Randolph dope covering and the raw places where I attached the fabric were sealed with the "green stuff" before I put the fabric on. It penetrated the wood to seal it and provided a compatible medium for the attach points.

Jeff Moore

I used EV-400 two part varnish on all exterior wood and systems three clear coat on all internal wood areas. I plan to use the Poly Fiber process and it calls for a two part varnish. I have heard of some guys using systems three clear coat for both interior and exterior surfaces but I figured I'd play it safe and use what the directions call for. The clear coat is compatable with T-88 if you are using it for your construction but I still taped off all areas where wood to wood contact was needed and came back with T-88 after removing the tapes. On the inside of my leading edges I used T-88 for the entire inside and that way I was assured of a very good bond plus good protection.

I'm getting ready to varnish some parts, primarily the outside of the wing center section. I have been using an exterior grade polyurethane for interior surfaces, but I have heard in the past that "special varnish" is required anywhere it will come into contact with the fabric covering/painting process as it may be exposed to acetone or MEK or enamel reducer or Laquer Thinner and that these will dissolve/remove/react with some varnishes.

Is this true? Do I need to use some type of epoxy varnish for the "exterior" wood surfaces?

Also, should I intentionally varnish the areas that will have fabric glued to them, such as my leading edge plywood wrap (I used ply instead of Aluminum on wing leading edges) or should I leave them "raw" or nearly so for the glue to stick better. Will the covering/painting process be adequate to protect the wood if it is not varnished?

Any experience/help?

Thanks