I’m working on the final coat of oil-based polyurethane for the cabinet doors, galley hatch, and berth doors as well as various wood filler spots. I mixed 50% polyurethane (commonly called Varathane – but this time actually is), 50% mineral spirits. This allows the urethane to penetrate the wood more. I’ll sand all of it lightly and coat with four or five more coats of water-based polyurethane.
It’s a lot faster if you roll it on and then smooth it out with a brush later. Rolling with a paint roller allows the polyurethane to penetrate more than with a brush but creates air bubbles and gives a rough texture. Gently, slowly going over it with the wood grain creates a much smoother finish.
Yes, you can actually put water-based on top of oil-based. It will typically bring out the colours and patterns of the wood more. You just have to make sure you lightly sand after each coat.
I’m using an exterior, UV-safe water-based polyurethane. It’s a little hard to find sometimes, but it’s better than adding expensive coats of UV block coating later.