ASHLAND OIL says: Simply scratch up dem "shot-up' spots real good. Then JB weld to fill'em within limits. Slow process.
Lightly sand smooth with old porcelon, finish with 360 paper all over the whole inside. This removes old soap (homeade) lye.
Wash down with CLOROX. Let set around the shop awhile.
Then refer to SouNdguy's idea. Check MSDS fer anything that will make your bottom HOT.
If OK in judgement........make a trip back to HOME DEPOT for info.
Most of us just use CLOROX(tm) as a weekly rinse anyhow. Use a rubber mat to protect BOTH male and woman bottoms.
John,PA CHIEF Susquehannoc tribes. REF: Mary Jamison White SQUAW.