If heat & acetone don't work, I'd try methylene chloride (aka Dichloromethane, MEC, DCM, di-chlo, or good old fashioned paint stripper) - evil stuff, will attack many plastics & rubbers, but it's good for softening most hard-cure epoxy & poly resins & won't attack most metals
Roughen up the surface to break the skin, carefully paint it on (I'd use MEC-based gel paint stripper & build a little bund with plasticine around the job to keep it off the rest of the board), leave it for 10 minutes, & scrape away the softened goop. Repeat 2-3 times if necessary. Don't go too far or you'll likely strip the soldermask & possibly start attacking the board material.