If you use the FT waterproof stuff, a quick brush down with steel wool and a decent can of spray (I use Krylon and Rust-Oleum 2X). I've been building with DTFB which benefits big from a coating of polyurethane (not water-based) first before paint. I've heard if you are light-handed with a few very light coats to start and you can skip the poly. I prefer to use poly as it adds some strength and also a good amount of waterproofing, but it comes with the price of increased weight. Just remember to hot glue all of your edges as well to improve the waterproofing. (Somehow I keep forgetting one area or another and I invariably get a separation of paper and foam. Last time I freaking forgot to hot glue the rudder joint and the blasted vertical peeled mostly off!)