Water-Resistant foam board is perfect for speed built kits like our latest sea plane the FT Otter. Water-Resistant Flite Test foam is a little different than the standard white foam board.
White foam board is easier to color with markers and can be decorated with lots of different materials, such as stickers and decals. However, white foam board is also more susceptible to moisture–which can quickly ruin an airplane.
Water-Resistant Flite Test Foam board is the same weight and uses the same foam core as the white, but a different paper coats the surface to help prevent moisture from reaching the foam core.
Building with water-resistant is the same as building with the non water-resistant, and painting the water resistant makes it almost 100% waterproof.
Painting water-resistant is a bit different than painting standard foam board. The surface needs to be prepped by sanding it so the paint adheres. We found that 320 grit paper works exceptionally well for this.
Use the same typical techniques for paint application as you would for white foam board. The drying process can be up to eight times longer depending on the type of paint and the temperature.
Take your time and let the plane cure fully before using the next color or masking scheme. 
Be careful when applying 3M 77 Adhesive Spray to ensure that no paint is lifted.
Happy painting!
What tips do you have for painting water resistant foam board? Share in the comments below!
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There is a product called Zinsser Bin Primer Sealer (white pigmented shellac) that comes in a spray can, and it is white. The stuff sticks to almost anything including water-proof foam board and any paint sticks to it. So, you can skip the sanding and spray a coat of Bin Shellac on the plane, then spray the paint of your choice. Since you are now painting over white, the colors really pop. Overall I think you will use less paint (less weight) by using the Bin shellac. You will also get a smoother finish since it isn't scuffed up.
Be sure to use the spray version of the Bin shellac. The brush on stuff is kind of runny and drippy so you won't get as nice a finished result unless you are good at brushing watery paints. It will work just as well though if you want to try it.
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What effects might it have on waterproofing the standard (ie. white) Reddiboard?
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