Woah, let me ask as to why do you need the FT stuff? I use clear spray sealer and put on two LIGHT coats after the parts are cut out and it'll be water resistant. I have found that the spray stuff protects against dew and light amounts of liquid. If your gonna fly off a pond just use oil based minwax. I use a paper towel and glove and wipe it on. They do make a spray minwax but no store near me has it and it's $10+ online. Water will bead up and run right off with oil based minwax just like a rainx'ed windshield.
But if you just want some water resistance for dew and damp weather, a clear spray paint sealer will work good enough. Dries in minutes, easy to use, cheap and available everywhere spray paint is sold, and works as a great base to keep the foam from soaking up too much paint and delaminating. Seal your edges with some hot glue or an iron and it'll last almost as long as your store bought planes.
WARNING - These spray sealers will soak into the paper delaminating it if you spray thick coats. Spray 2-3 LIGHT coats. If you spray before you build by the time your on your last part your first part will be dry enough to do the 2nd coat. Try testing it on some scrap parts first, get your technique down pat before you do actual parts. Feel free to test your tests in a sink. Works surprisingly well.
Anyway, just an $$$ saving idea for you. I know spending $130 on FT foam isn't very good...
Edit : This is what I use. Found it at Walmart for $4.
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