I can't see the point of embedding copper in a resin or a varnish - about as much use as trying to lick a choc-ice without opening the packet.
The cheapest solution is to save all the copper piping you were going to throw in a skip (or take to the scrappie), straighten it out, hammer it flat and bend it into squares or hoops - your slugs will be bashing their heads against active copper for the next thousand years...
Cheers.