Maybe hit the caulk with a toothbrush and see if the varnish let's go. Stainless steel wool perhaps?
In any event, varnish isn't going to seal well to caulking, which means any cracking between the caulk and plank will allow water in and that's the death of varnish. Most things I see varnished, do not have caulk joints. Even without them, varnish is hard enough to maintain.