The interior finish of this an old wood canoe appears to have the original clear varnish with a second old yet later clear coat of something unknown . The second coat has become checked and dark brown . The usual chemical paint stripping agents seem to turn these finishes into a thick slime which is not water soluble and completely plugs up the cleaning tools . It leaves a chewing gum type of residue . I have tried several solvents to no avail . Laquer thinner and acetone do work somewhat but not well enough . I'm wondering if the second coat is shellac ? Any ideas or assistance greatly appreciated .