Try and scrape off what you can then give a whirl with mineral spirits. If you don't want to remove any of your old paint be gentle with the scraper
- a no brainer - . The spirits shouldn't attack the existing paint unless you pour it on and leave it set too long. Might get some alligatoring out of it if you do. If you don't like the mineral based route try something with citrus in it, like Goof Off or the like.
