: Hi, I'm competing in a cardboard boat regatta and I bought the Helmsman Spar
: Urethane to apply to the cardboard. Now, the cardboard has already been
: painted so I'd be applying over that surface. Is this the right choice?
: Does anyone have any experience with waterproofing cardboard
: (understandable if not)? Thanks.

Depends on the rules for your competition. For the competitions I've been a part of, rubber paint works the best. This is paint used for roofing on trailers, etc. Make sure to seal all the edges with caulk or something else. Also make sure you waterproof the inside. Contact cement is a great material to glue layers together as it is waterproof. Liquid nails also comes in handy.

Jon T