How do you know when you've sanded through the varnish?

Builder's Forum
1

So I made a Chesapeake 17 a while ago, and I completely finished the boat (including 7 coats of varnish) before I looked behind me and realized "oh yeah, I should put that compass mount on the boat itself".

So now I'm sanding through the varnish in order to get down to the epoxy coat so that when I epoxy the compass mount onto the boat, it'll actually stick.

My question is - how do I know when I'm through all of the varnish?  Will the dust change colour?  Will it start smelling different?

I've been using 120 grit paper, and so far the dust looks a little on the "yellow" side which I've convinced myself means it's still varnish, but I have no idea if that's true.

Any suggestions?

Thanks!

2

What works for me is wet sanding with #400 paper. While I'm still sanding varnish, there's a kind of soapy feel to the wet surface. One the "soapiness" goes away and the paper gets a bit of a bite, that means I'm into the epoxy.

If your nose is sensitive enough, you can also pick up on the difference in smells while wet sanding. If you're dry sanding you should be wearing protective gear to keep from inhaling the dust.

And yes, dust color also indicates what you're sanding. Yellow is varnish, white is epoxy.

Good luck,

Laszlo