Technical Counselor Notes
During the past four and a half years several members of EAA Chapter 1279 at French Valley have been building a Pietenpol Air Camper as a group project. The airframe is nearly finished and ready for covering. We will be covering the airplane using Poly Fiber covering material (the Stits process). I would like to share a few thoughts on what we have learned about varnishing using Poly Fiber's two-part epoxy varnish. There are two things in particular that are absolutely critical to obtaining the best results. The first is following the mixing instructions explicitly. The second is making sure that the environmental conditions are right for proper curing.
Southern California is known for having great weather almost year round. When the sun is out and the temperature is delightful, it is easy to ignore the humidity in the air. We found ourselves having to do things over again because we made the mistake of deciding that we were ready to varnish, instead of asking ourselves if the conditions were right for varnishing. Not allowing enough induction time for the conditions will result in a tacky finish which will never fully cure.
Be honest when evaluating the conditions. The instructions give temperature guidelines and say allow additional induction time for high humidity conditions. My suggestion is, unless you are on a production schedule, if there is humidity in the air, wait for a better day. There are always other things you can work on.
When mixing, DO NOT fudge on the induction time. Mix Part A and Part B, then find something else to do for AT LEAST 30 minutes (45 minutes to an hour would be even better). Then, and only then, add in the proper amount of Reducer. Again, do not fudge on the Reducer. The Reducer is a key element in the proper curing of the varnish. By the way, use a coffee can or glass container for mixing. The varnish will eat right through a Styrofoam or paper cup.
The instructions also say to apply two coats -- a third coat is optional. We achieved the best results by applying the first coat, allowing a couple of days curing time, then sanding with 150 grit sandpaper before applying the second coat. The bare wood will soak up a lot of varnish during that first coat. Sanding is necessary because the first coat of varnish raises the grain of the wood.
Now, apply the second coat. You can consider the second coat the finish coat, if you are the type of person who figures that "it's all going to be covered anyway." If that is your mind set, adding a third coat would only add weight. Forget it. Two coats is all you need to seal the wood before covering.
However, if you are a craftsman and you want a silky smooth, glossy finish, then sand that second coat almost completely off using 150 grit sandpaper. The surface should have an evenly dull appearance and be very smooth to the touch. Then carefully mix the final coat according to the instructions, but add a little extra Reducer after the proper amount of induction time. The varnish will seem thin and runny, but it will cure to a hard, glossy finish. That's the ticket.
Try these simple tips the next time you varnish using Poly Fiber's two-part epoxy varnish. You will love the results.
Steve Williamson, President
EAA Chapter 1279
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