how thick can I "skim coat"?
Any kind of build up as you're describing would be best with a coat or two of setting type compound then skim with regular mud. You can put a fairly heavy first coat on as the setting compound doesn't shrink much. Thinner coats for the skim(s). Can you sand (grind) any of the high spots down to minimize how much buildup you need?? Pics might help, as Beerdog mentioned........
If you can read this, thank a teacher. If you can read this in English, thank a soldier. Support our troops.