Hey SR....   If the varnish is on top of the paint, it is a simple matter of sanding off the paint, down to the varnish but, if you can control it, NOT thru to the 'glass overlay and plywood.  If there is an epoxy coating below the paint & varnish layers, you can just sand down to that (epoxy) level.  If, by accident, you sand down to the 'glass cloth, just apply more epoxy, ALLOW TO DRY, feather and paint & varnish again in those select areas...   ~TUGBRUCE~

Even with tape, it's very difficult to get a clean line between varnish and paint. I solved the problem on my Merganser by applying a strip of 1/4" vinyl tape along the seam. It gives the boat a very professional look and still looks new after a full season of hard use. -Wes

I fell your pain. When I start getting anal about the paint and varnish lines, my neighbor kindly reminds me that "it's just a rowboat, get over it." I find that the varnish will rub off the paint if you get to it soon enough, being careful to not mare the varnished surface adjacnt to the area to be cleaned up. A wee bit of really light sanding (steel wool and tack cloth really), just enough to remove the varnish and then touch up with paint as needed. Ohh, and buy a really small paint brush, half inch by pencil thin - great for touch ups and fixining really small blemishes.