I wet sand the varnish either 180 or 220 depends on what I've got on hand at the time. All you're really looking to do is to flatten it and take the gloss off. But if it's too smooth the new coat of varnish won't have a decent surface to bond to.

Get a squeegie, it's the easy way to see if you have unsanded areas when you're wet sanding, dries almost instantly that way.

Bill H.