I hit on a trick to make removal of old foam backing from the headliner form much easier.
I had a blue headliner out of the parts car I'm dismantling presently and wanted to strip it to re-cover in burgundy for one of my other cars. Did some thinking and decided to hookup the paint gun to the compressor and fill it with Methyl Ethyl Ketone.
Use in a well ventilated area. - Methyl Ethyl Ketone vapor/air mixtures are explosive!I removed the fabric (color) portion first. Then I sprayed out a small section of the foam and let it sit for a couple of minutes to soak in and help release the adhesive. Repeated this by sections about 18" square at a time until done.
I was able to pull off huge pieces of the foam backing easily without damage to the fiberglass form. I was quite pleased with this as my previous experience doing a headliner was a very nasty mess trying to strip the old material in small (nickel to quarter sized) pieces and ended up using a torch to burn off some stubborn foam. This would be most effective when the foam hasn't yet broken down into the partially liquefied, tar like state that occurs with advanced degradation of the material.
I would let the form sit outside for a day to ensure all the Methyl Ethyl Ketone has evaporated and won't smell up the cars interior before recovering with new material. Just thought I'd pass this along.