As I mentioned in part 1 of this series of articles, the use of oil-based Minwax polyurethane was a completely new build process for me, so I wanted to do some experimenting before I got the plane together in case there were issues. I'm glad I did as it helped me select the right product and have the confidence that I was applying the product correctly without adding too much weight or other adverse affects to the foam and structure of my plane. Here is Flite Test's video on how they recommend using Minwax to waterproof and strengthen DTF. The timing on how long to leave the Minwax on the paper before wiping it off is not very clear, so that was one thing I wanted to experiment with.
Also, I wanted to see how the Minwax affected the paperless foam and hinge tape as I didn't want the tape to be weakened and have control surfaces come off in flight.
I found the Minwax very easy to spread on scrap foam. After experimenting, I found that leaving it on the paper for at least 90 seconds to 2 minutes before wiping it off with paper towel saturated the paper sufficiently that after drying water would bead and run right off the surface of the treated paper. Again, put the paper towel you use to wipe off the Minwax outside in an area where the odor won't cause issue and ensure it is not in contact with too much heat as it is flammable until completely dry.
I also experimented to ensure it didn't melt bare foam, it did not and it added a little strength to the bare foam, but I don't think I would apply it to all the bare DTF just to keep the weight down.
I spread it liberally over some pieces of Scotch brand Tough transparent duct tape that I use for my control surface hinges, left it on for 90 seconds to two minutes and did not notice any wrinkling or bubbling of the tape or adverse affect to the adhesive. As I accumulate flights on the plane, this will be something I will monitor for durability.
I didn't really notice too much appreciable weight increase on the scrap pieces of foam that I used, so wasn't too worried about that. It is important to wipe off as much of the Minwax as possible when applying it to the finished plane so that it doesn't accumulate in corners or seams and add unnecessary weight.
Since I applied the Minwax after the air frame was completely assembled and finished, in hindsight I probably should have tested on some scrap to see if the Minwax affected any of the glues that I use, however, everything seems to be just fine. As mentioned in the video, even after the Minwax has been wiped off and is completely dry, it has a non porous surface that is not conducive to holding any type of adhesive, so all the gluing needs to be done before Minwax is applied or if doing the parts before hand, the areas where glue needs to be applied would need to be carefully protected with tape.
I think it is easiest to construct the entire plane first, have all the sanding and finishing done and then apply the Minwax before paint, essentially like using the Minwax as a primer on the areas where the paper has been left on the foam.
In the next article, I will discuss the actual build.