the interiors were completely shot to begin with :-(i wouldn't ever remove original paint unless it was absolutely necessary for the wellbeing of the drum, lest I be crucified by the Gods of vintage mojoism :-p
Hey, here's the key: "...completely shot to begin with..."
I interpret this to mean these are too far gone to actually restore anyway, but can be re-done with the "player" in mind.
Here's what I do with these types of things:
1) I lightly sand down the interiors to knock off whatever is loose and to rough the rest of it up to give the paint a good surface to adhere to. If there's any lifting or cracks in the interior ply I either re-glue & clamp or use Rock Hard water putty (and sometimes both).
2) I then repaint with plain, simple Krylon paint that you can buy at Wal-mart or any hardware store. For Ludwigs I use White Satin (not gloss, semi-gloss, or flat). For Slingerlands I use Pebble Satin (again, not gloss, semi-gloss, or flat). These colors are NOT exact matches, but close.
3) I don't worry about coverage, as both Ludwig and Slingerland were at times very inconsistent in the thickness applied, as someone previously mentioned. After all, we're talking player's drums here, not vintage museum pieces.
4) It depends upon how many drums and sizes, but I've found one can will easily cover a standard 22/16/13 set.
Here's a thread that shows some interior work that I've done:
http://www.vintagedrumforum.com/showthread.php?t=27167
Hope this helps,
Bill