i took mine to 220 and it was fine.  Other folks here have recommended 320 but it seems like overkill to me.

Your final sandpaper grit selection is a balance between time you want to allocate, desireability of making your boat more beautiful and shiney as your coffee table, the "relative" amount of abuse you will subject your boat to, and your tolerance for living with scratches.

I have defaulted to a two-part solution: 400 grit for deck (I am not banging my kayak up with a lot of "T- Rescues") and topsides and 220 on bottom, where the extra effort is not worth it as I can't avoid the scratches from the beach.

When I go to 400 it by way of 220, then 320, hten 400. I will defer the use of 1000 grit to those capable of constructing a more high quality craft than I.

Good Luck!

Tim Clark

a Quick note at a recent "Major" antique and wooden boat show the Concours winner gave these answers to your questions 320 grit wet  as last step before varnish and 320 lightly between each additional coat You probably do not want to know how many coats of varnish these finishes take but it is many and just so the know everything builders with work boat varnish that claim it always scratches before I have finished my 1st use have something to ponder

his comment to a question of durability was that if you give the right varnish (his was Epiphanes) enough time to really finish drying 2-3 weeks you will have a very hard durable varnish

Now just to let you know this gentleman  has won over 40 awards for Concours finishes and this shows boat has won 3 awards in 3 years without refinish and although it lives in a boathouse it is out on the water at least weekly