Now that the floats are built, the next step is to apply some kind of waterproof finish. There are several options. What Chuck Cunningham used do was nothing. He would just leave the foam naked, or at most he would apply fiberglass packing tape. My 36″ floats are encased in balsa wood, and my 27″ floats are all wood, so I needed to put something on them to keep the water out. The ones I made a long time ago were finished with Monokote, which worked well. I also made a small set of floats with foam cores, balsa sheeting, and tissue paper stuck on with Aerogloss dope, which is water and fuel proof, but you can’t get it any more.
This time I decided to follow a procedure I used to finish the speaker cabinet of my harmonica amplifier.
This speaker cabinet is covered with cotton fabric stuck on with Minwax Polycrylic. I wanted to use similarly crazy fabric on my floats, but I couldn’t find anything light enough in my wife’s quilt shop for a model plane. Actually, I probably could have if I wanted to dig all afternoon, but I got tired of it and decided to use brown paper instead.
The two companies I order balsa from stuff the empty space in their shipping boxes with brown paper. One company uses heavier paper, and the other uses lighter paper, so I used the lighter of the two, which is quite nice for the application. Boxes from Sig are stuffed with really nice lightweight white paper. I also considered newspaper, which is the perfect weight. It would have looked great, but I couldn’t decide which page I liked best. Should I go with the advertising pages, or the church news, or the police mugshots? That was a decision I was having too hard of a time with, so I went with brown paper instead.
Minwax Polycrylic is a water based wood finish, which is very easy to handle and clean up. A quick internet search revealed that it is water resistant, but for outdoor applications you should use oil based polyurethane or spar urethane. I’m able to set cold drinks on my amplifier with no problem, so I can attest to the water resistance, but maybe a float in a pond is a different story. I want these floats finished with polyurethane just to be sure there won’t be any trouble. Testing with scraps of wood and oil based polyurethane revealed that oil based polyurethane does a poor job of sticking the paper on. Water based polycrylic permeates the paper and sticks it on nice and tight, so the plan is to stick all of the paper on with polyrcylic, then sand the shine off of it and finish with a coat or two of oil based polyurethane. That’s the best of both worlds.
To start, I cut a strip of paper the right width and set it aside. Then I brushed a heavy coat of polycrylic onto the bottom of the float. The paper was submerged in water, then it was applied over the coat of goo. Bubbles were squeezed out, then a coat of polycrylic was brushed over the paper. Care must be taken to get the paper crammed all the way into the inside corner at the step without tearing it.
I didn’t want the first piece of paper to overlap the corners, so I cut it to cover only one surface. I would have liked it to be closer to the edge, but I didn’t feel like cutting another strip, so I just went with it. My other three floats look better than this.
After doing all four float bottoms I set them aside to dry for about 4 hours. Then I applied paper to one side of each float. Once again, a thick coat is brushed on, the wet paper is laid in place, bubbles are pressed out, and a coat is applied onto the paper. This time the paper needs to be trimmed at the corners, sort of like applying Monokote. The overlap zone is coated, then the edge is brushed down with more Polycrylic.
You have to make sure all the bubbles are gone, because you get one chance with this stuff. It’s pretty easy. You just have to take care.
So far I’ve done everything but the tops of these floats. Stay tuned for more photos, and I’ll show you how the finish turned out, and then I’ll show you a really great idea I came up with for attaching the floats to the plane.