Hi Tony,
Welcome to Mayhem and model boat building.
Depending on the type of hull it is you have some choices.
1. If it is a Fibreglass hull is doesn't really need waterproofing but a lot of people give the insides a coat of paint to cover up all the epoxy, car body filler etc used in the building and make it look neater.
2.If it is a wooden hull, whether sheeted or planked, once you have done your hardware installation (Most things stick better to wood than varnish) Then you can give the whole inside a coat of varnish or paint to help protect the bare wood. Some model kits and plans recommend that the inside is skinned during the building process, with Fibreglass or tissue and then coated with GRP resin, Epoxy resin or Acrylic, This adds strength and waterproofs it.
In wooden plank on frame hulls, a layer of resin will help to fill any gaps and further bond the planks and frames together so this is to be recommended. This is usually done with a membrane of some sort like nylon, glass cloth or tissue embedded in it.
3. Plastic or foam hulls, These don't really need waterproofing as they are already waterproof. A coat of enamel paint can be used to cover up any messy installation issues. (Do not use Cellulose car paint or similar)
Hope this answers your question.
Ron.
