The purpose of the water is to keep the sandpaper from gumming up, so the details of how you apply the water and how much water are not critical. I dip the sandpaper in a bucket of water every few feet of sanding (I do it by hand) and slop on some water from my hand and the sandpaper as I start sanding again-not very scientific.

Also, the grit need not be 400. It depends on how drip/sag-free and dust-free your varnish coat is. If you've got some sags, you may need to wet sand with 220 or even 180 to level it. A lot of dust may also justify 220. I use 220, 300, 400, and 600 on successive coats. Use thin coats - way easier to be careful in application than to deal with sanding sags and runs. I apply mine with a disposable foam brush - never had a sag unless I fail to take my time.

Jim

: I am at the point of applying the first coat of varnish (atop epoxy) on a
: boat project.

: I've (dry) sanded the West System epoxy down to a drip/sag/run free finish.
: After the first coat of varnish is on and fully dried I understand that I
: should wet sand with 400 paper before applying the next coat of varnish.

: Question is - I've never wet sanded before. Can someone give me a quick
: primer on wet sanding. (ie, how wet...what is wet, the boat or the
: paper...how to clean up the hull before the next coat of varnish...etc,
: etc, etc.

: Many thanks. While I do believe in learning from one's own mistakes, I've got
: too much time and money in this boat to screw up now :-)