Carpentry and Woodworking - Wood countertop for bathroom

I am building a bathroom vanity form ash wood and would like to know how to put a waterproof finish on the countertop.

Here's my take on the problem. (I built a wood top once). You have two problems - water that is spilled and moisture or humitidy. the top can be "sealed" with several coats of polyurethane to protect it from spills. This should include all end grain, including any cutout for the sink as well as the underside. However, you should know, that no finish is impermeable to moisture. Sinks are made of stuff like plastic, ceramic, cast iron, etc - all materials that do not "move" with changes in humidity. Wood does move. You will have a constant problem maintaining the seal between the sink rim and the top.

HI,
Couple of tips, use a polyuretane exterior wood glue. When you fasten the top down to the base cabinet elogonate the holes to allow the top to move, you don't need big slots, just a little And I would myself try a marine grade epoxy finish on the top, and Dave is right, you need to cover all sides, ends and cutouts. Use a good grade of silicone sealant for the fixtures.
good luck,
Trent