Best Finish for Kitchen Sink

ditto ^^. Any polyurethane should do.

If you’re going to use an exterior varnish, make sure it’s not going to leave your surface too “golden”. In my experience exterior varnishes are more yellowing than interior-only varnishes (that is they start yellow, not they go yellow over time). It could be to do inhibiting UV, which is obviously a much bigger problem for outdoor wood than indoor, but I’m not exactly sure.

Exterior varnish also stays softer when it’s cured, to cope with wood movement so I’m told, which would make it very liable to pick up scratches if used on a worktop. In fact this should be more of a worry than the occasional splash. If you can get a varnish designed for floors, I’d go for that.

All the oil-based polys should be perfectly waterproof, even the bog-standard interior-only. I’d use an interior poly on the cabinets, whichever one looks the best value.

I’d also be wary of slapping on too heavy a coat. I’ve heard people talking about “flooding on” finishes like varnish and gloss paint, but, again in my experience, thinner coats and more coats always leave you with a better finish. Especially is this true with polyurethane, which looks unpleasantly “platic-y” when badly applied.

The joiners’ workshop I worked in for a while would apply three coats for interior work, the first coat thinned down half and half with white spirit, then the next coat 70/30, and a third coat unthinned. Further coats if the finish looked uneven, but three was almost always enough for a consistent finish. Don’t take this as gospel though, ask ten different people how to apply varnish and you’ll get ten different answers.

Another thing to bear in mind. Though most polys take around a day to “dry”, it takes weeks before they’re ready for rough treatment. Your worktop surface will be liable to pick up permanent scratches in the surface for quite some time after the varnish feels dry.