sanding is to give the next coat something to adhere to as well as smooth the surface from drips or other imperfections before building them up even worse.

wet sand with 220.  wipe clean with wet paper towel to get rid of the dust that is in the water.  once hull is clean and dry, make a final wipe with mineral spirits to remove any final contaminants.  let dry (mineral spirits should evaporate quickly -- 15 to 30 minutes.    then do the next coat of varnish.

this procedure may appear to add some time....repeated 4 to 5 times puts a great finish on the boat.  be patient during this phase....you are almost there.

by the way, for this phase, paper towels are fine and better for avoiding contamination.

best

howard

Because "varnish" is becoming a generic term for a number of products, you might want to provide us with the brand name. Also revew the manugactrurers advice.

Most varnishes are best sanded with a 220 grit paper or pad to rough up the surface for better adhesion and knock down high spots before they build up to be noticeable.

Preperation of the surface is very important. Also using a supply can for the varnish being applied and keeping the larger store sealed is important for getting the maximum coverage.

Defender How toVarnish Wood

West Marine Do-it-Yourself: Top Tips on Varnishing

I have always used 400 grit to wet sand varnish. I keep about 3 gallons of water in a 5 gallon bucket to rinse in, and a spray bottle to wet the surface. I also keep an epoxy spreader with a piece of neoprene on the edge to use as a squeege. It works well to remove the sludge as you are working, and will reveal any areas needing more sanding. Rinse the spreader frequently and you will have a pretty clean surface thru the sanding process.