Sanding thru the varnish is a possibility and is to be avoided. It usually takes a couple coats to cover when doing maintenance coats, so don't over sand and let the varnish fill in the low spots. Sometimes 3-4 coats are necessary if the the surface is rough. The heaviest sandings are before the first coat and before the last. Spot coats on places without varnish can be helpful. 220 grit is fine for all coats unless you tend to over-thin the varnish. I use it straight out of the can unless the weather is real hot or the boat is real big.