I have two boats with 4mm hulls and I treat them like I would any commercial fiberglass boat. The decks of the boats are 3mm, however, and I try to be careful with them. If I were to build a 3mm hull with, say, a single layer of 6oz. glass on the outside, I would try to handle the boat carefully. I don't think I'd use 3mm for a boat to use every day. I might if I wanted a lightweight racing boat that would be used under more controlled conditions.