The bottom line is that your new paint may not be compatible with the old paint. If you don't know what it was originally finished with, you'll probably need to strip it. Some MEK or acetone on a paper towel will remove dopes and most enamels easily enough. If you find that no chemical solvent will attack it, it will need to be sanded. If you can verify that it was originally done in epoxy paint you can go over it with nearly anything you want, although doing so adds the weight of two paint jobs instead of just one.