This is a long shot, but I'm trying to replace the water pump on my wife's 1988 Toyota pickup. The guy who put the last pump on used a TON of RTV liquid gasket to seal it on, and it might as well be fucking welded in place for all it will move. It's an aluminum engine, so I'm trying not to attack it with a chisel or anything too violent. Any comrades here with car mechanic experience got any tips? I'd prefer not to drop the $400+ the mechanic is going to charge to do it.
Not sure how well this will work, but after chiseling away the excess goop (not aiming the chisel/blade between the mating surfaces, just going around the outside to remove excess) I wonder if you could "floss" it with a thin strand of steel wire, or a feeler gage. This will hurt the aluminum, but like @BearOfaTime@lemm.ee suggested, if you break the adhesion on one side it might encourage it to come free with a mallet, leaving most of the sealing surface intact.
If the sealing surface does get marred a bit, you can smooth it out with a wet, fine grit sharpening stone. It won't be quite as flat as sandpaper on a surface plate, but it can fit in a tight space (and costs an order of magnitude less). We use these on our CNC machines to smooth out flat surfaces when they get dinged up (like the table of a mill or wire EDM). They don't really remove voids though, just protrusions.