I've had good luck with Helmsman Varnish
in the same application. It is fairly inexpensive and available at most hardware
stores. You can use more expensive marine
grade varnishes, but if you can keep the rudder
and daggerboard out of the sun when the boat is not in use there is no need for a more expensive product.
As for the procedure, you will want to remove all of any existing finish that is on both pieces, typically with a heat gun & scraper and then sand to ~120 grit before starting to refinish. Put on as many coats as you can, at least 3-5 coats per the directions on the can. I have found that hanging each by a string allows you to varnish
both sides at the same time.