Carl,When you say you varnish to prevent creep do you literally mean you varnish over the edge of the tape and then paint next layer over the top of the varnish as presumably there will be some varnish over the edge?
I only use acrylics, so I can't say how this works with enamels. When I've finished masking, I use a small brush, and go over the edges of the masking, paying attention to panel lines, dimples etc. I use a slightly watered down varnish, matt, or gloss, depending on what the undersurface finish is, the thinnest coat, just enough to seal the mask edges, & allow to dry. I usually spray, when I use a mask, but there's no reason that it wouldn't work with brush painting. I've only ever used standard varnishes, some of the fancier polyurethane varnishes may not work, and could lift the paint.....................
It's not always perfect, but it reduces the amount of touching up needed, obviously the smoother the surface, the better the chances
Dave