The paint in our tub has been developing air pockets under it for a while, and this morning a big chunk ripped off. 
As far as I can see, the layer underneath is undamaged except for the stains. Might it make sense to strip off the rest of the paint and live with the stains until I have the time and money for a better repair?
For those wondering how such a big chunk came off, last night I took off the non-slip mat which was suction-cupped to the tub and scrubbed the tub (non-abrasive cleaner, but rubbing pretty hard). This morning I stepped in for a shower, and the area where the mat had been detached.
Best Answer
I'm not sure if there is a correct answer. Stripping off all the paint would be a job, especially where it may be well adhered. I think you would be well served to remove any loose areas as they develop until you are ready to do a complete refinish. When you do the refinish, then it will be necessary to strip all the paint and etch the original porcelain before proceeding. .