You've got a big contract on your hands getting that core off the skin. Problem is the core is bad enough to need replacement but good enough to still have a strong bond to the skin. Maybe use a sanding grinder with 30 grit for the tough areas?GF, You sure he should use 5200 to seal the panel? If he does, it'll be impossible to get the panel off if he has to get in there again.
Good progress! Core material is a chore to pull off the skin if it isn't completely rotten. I think if I had mine to do over again I'd simply make a whole new panel with new skin. Probably could make one in half the time as re-habing. Laminating the new panel to the skin, best way to go would be to vacuum laminate if you have the set-up. I used weights which worked semi-OK but there are some voids under the skin where it didn't bond. Vacuum will give you the best results, no question.