Good question Barry. Having done some refitting work recently I'll say it's not easy to tell. I find it scraping more controllable than sanding. If scraping or sanding, carefully note color of first varnish shaving/dust. It'll probably be a bit yellow. Epoxy will be slightly more white. I ended up taking a good bit of epoxy off and it gets easier to tell as you get closer to the wood. It's easier on the fiberglassed areas, just scrape down to the weave. At any rate, if you're adding more epoxy and fillets and varnish it'll be hard to take off too much. Just stop when you get to the wood ;-)
Holler back if you’d like to see some scraper tools that have worked well for me.
Cheers,

e

I WANT TO SEE SCRAPER TOOLS THAT WORKED WELL FOR YOU!

Uh, you said to holler...  

I love scrapers, all sorts from cabinet to babbit.  Always looking for scraper ideas.  Thanks!

Mike

Save your old, worn out files. Great steel in 'em for making custom scrapers as long as you have access to a bench grinder! I used to look for old files while at flea markets & antique stores. Good for making lathe tools too if you're ambitious enough to be involved with turning at all.

You'll need a propane or MAPP torch though if you decide you need to re-shape the working end before forming your new scraper edge. They don't bend easy unless glowing cherry-to-ruby red.

Oh, and for the OP: lacquer thinner or acetone will dissolve most varnishes pretty darn quick, leave epoxy underneath untouched. Modern paint removers (sans methylene chloride) will too and may be safer if you're at all uncomfortable about taking necessary precsutions for handling flammable solvents.

Varnish scrapings will dissolve in those solvents, epoxy's won't.

Really old, weathered vatnish scrapes to flakes or dust. Newer stuff tends to curl a bit, scapings may even stick together. Cured epoxy's scrapes to dust too but usually takes more effort than varnish due to its hardness.

With the Schooner Gold varnish that CLC sells, I can tell by the feel when I'm wet-sanding sanding varnish and when I'm sanding epoxy. Varnish feels soapy, epoxy doesn't. The smell also changes when the material changes from varnish to epoxy.

Laszlo

   Thanks all. It is good to hear my thoughts on shaving color is a good indicator and I will try the acetone test. After years of construction my nose doesn't have the sensitivity to note the change but wet-sanding may be a good option along with the scraper. I will try to post some photos, or a link, when completed.   Barry

You got it Mike! I'm getting to prefer scraping to sanding. Cabinet scrapers work great but dull quickly. Carbide lasts and lasts. Until you hit a screw head.

My favorite-

It's a cutter from a helical jointer head screwed to a scrap of cherry.

very precice.

For heavy work this is awesome and it has good control-

Just ok in use but you don't have to make it-

A putty knife with an edge. The business end is filed and honed to a flat, straight 90 degree angle. Great for drips and runs. You'll need to make a jig to develope the hard 90 angle.

Sanding with a Fien tool. Spent blade sandwiched in foam with contact cement and sandpaper stuck to it with spray mount-

There are a few more here. Click the thumbnail to read captions with more complete info...

Happy scraping!

 Thanks!  I don't even mind sanding, but scrape EVERY WHERE I can.  

I've got tons of carbide inserts for metalworking, never even considered using them for woodworking.  Brilliant.

Mike