That is, polyurethane that has not been treated, sealed or waxed. Other surfaces don't give the polyurethane enough grip for it to survive for very long. If you use quality paint you do not need to apply any protective finish over it. Latex paint is not a great choice for any surface that suffers possible abrasion, such as a countertop. I also loathe using latex paint on bookshelves or other surfaces that are going to have objects placed on them.
Alkyd paints are the finest of the oil paints.
They dry very hard, are washable and about as abrasion resistant as you can get in a paint. The only better paint you could apply in my opinion would be epoxy paint, which is a two-part paint that chemically sets instead of drying. However, they are not as easy to find and quite a bit more expensive. I have used polyurethane paints a few times, but I am not convinced they are really any better than alkyd paints in durability. With care and gentle treatment, the painted surface should last years.