I am doing a repair to my coaming lip on my west river 180 which i cracked during rolling practice.

i was getting out of the boat and bracing behind myself with a paddle that apparently was on the coaming lip and when i put my weight on the paddle, it cracked the top layer (the lip) and pulled part of it away from the coaming riser pieces (which were not damaged).

its an older boat and i apparently never glassed the coaming lip/inside of the risers) but just put epoxy over it and than varnish over the epoxy.

the approach i am looking to take is 1) to sand the varnish off the coaming lip, 2) fix the crack/re-attach it to the coaming risers with some cabosil-thickened epoxy, 3) glass the entire coaming lip/inside edge of the coaming....and 4) fill coat and varnish.

my question is, without sanding all the way back down to the wood on the coaming lip, is there a reliable method that tells you you are through the varnish and into the epoxy? how do you ensure there is no varnish on the old epoxy prior to the reglassing effort? is there a chemical test, a heat test or some way to confirm you won't try to put epoxy over varnish?

many thanks in advance for your help. any other comments on the repair approach welcome as well.

howard