How do I go about applying Spar Urethane/Polyurethane to a cardboard boat?

Question

Should I use a roller to apply it or a paintbrush? Are many thin coats or less, thick coats recommended? The cardboard involved is already painted, does this matter? The reason I'm asking is because I am building a boat for a cardboard boat regatta and we are using a new waterproofing method that we aren't very certain of. Has anyone waterproofed cardboard that maybe has a better alternative than Spar Urethane? Please answer ASAP.

Answer 1

i would spray it on or brush it on

Answer 2

The method isn't as important as the application itself.  A brush might leave behind some stroke marks, though a roller may leave a somewhat mottled finish.  I suppose though since we're discussing a cardboard boat, that hydrodynamics are not particularly vital.  It's more important that it simply float.

I would start with one thin coat to seal the cellulose of the cardboard, not too thick... but just enough to wet it.  After that it shouldn't matter too much how you apply it.  I would recomend some form of sanding between the coats however, it should improve the adhesion between the layers.

Good luck, and happy boating!