Thunderpilot

Hey all,

I am building a Cloud Clipper flying lawnmower from plans. The plans/instructions called for mostly foam for the build and did not specify which type. I went with 3" thick 1.3# EPP foam for the primary airfoil. I built my own hot wire cutter which worked pretty well. Now I am at a point to finish the airfoil, i.e. filling and sanding some of the bumps and pockets that are bound to come up. However, in sanding the EPP foam even an 80 grit sandpaper does not seem to do anything to the foam, the sanding bar seems to simply run over the foam with no change in the surface texture.....hmmmm. I tried looking on the internet with some explanation as to how to sand/finish epp and prepare for covering but am coming up empty.
Any help appreciated.

Also, based on the plans/instructions I am planning on covering the foam with Sig silkspan covering by adhering it with thinned alphetic resin and then painting with acrylic paint - this already seems like a big pain to me and I haven't even started! Any other covering recommendations?

AviationNut

I've had good luck sanding EPP with drywall sanding screens in the past. I've also covered an EPP plane with Ultracote with good success. First, I sprayed the plane with 3M #77 and let it dry, then covered it with low heat on the iron - test on a scrap piece to get the heat correct (too much will melt the EPP). Just a light misting of the #77 is needed.

You can also use colored packing tape to cover the airplane (the spray adhesive is still necessary). This is much lighter, but doesn't look as good.

Thunderpilot

Hey AviationNut,

Thanks for the advice. Putting on Ultrakote sounds a lot better than the old school covering! Did you use a heat gun to shrink up the ultrakote or will that be too hot for the foam?

Also, I am planning on using some of the blue or pink insulation foam, ala Home Depot to make some of the other small pieces I need. Can I use ultrakote on those as well?

Thanks again

AviationNut

You won't need to shrink up the covering, just pull it tight while ironing it down. It sticks like crazy to the spray adhesive. You could use a heat gun, but you'd have to be very careful since the foam will easily melt. Not sure if you could iron Ultracote on the pink/blue foams. They melt pretty easily, so you might want to use the #77 on both the foam and the Ultracote and contact-cement it on (especially if it's just flat surfaces). Or maybe paint them directly with an acrylic.

JustErik

I'm just about to start on my first ever EPP plane, a TufFlight Bouncer. The TufFlight guys say sanding is best accomplished with 40 grit paper mounted on a solid sanding block. That's what I plan to try. But if it doesn't work too well, I'll just leave the finish as it. I plan to beat the plane to death practicing low and slow 3D stuff anyway. I'm too chicken to risk my bigger planes.

Thunderpilot

Hey JustErik,

Thanks for the post. I think I may give the 40 grit sandpaper a go and see how it works. I went out and got some 80 grit drywall sanding screen and the tool to hold it. The screen material does a pretty good job but is pretty expensive compared to a sheet of sandpaper - it was about $20 for everything. Have fun with the Bouncer - looks pretty sweet!

Thunderpilot

cutaway

Its not as hard as it seems, just messy. The silkspan over foam adds a lot of rigidity you won't get with plastic coverings. Just silkspanning a foam wing's leading edge up to the spars makes it almost as torsionally rigid as sheeting would. A 2" wide strip of sinkspan wrapped around the T/E will help keep it rigid.

Also usable with the thinned aliphatic technique is dollar store gift wrap paper. This is very common on some foam C/L combat models.

Thunderpilot

Thanks for the info Cutaway!

Thunderpilot