Lye is my knee-jerk response. And yes, it most likely will eat the paint. But it won't harm metal (well, not ferrous metals in the reasonably short amount of time you'll be using it, up to maybe a day of time to work if particularly difficult). The easiest form for this may be caustic oven cleaner spray. If it says oven cleaner, it has lots of safety warnings, and says not to use it in self-cleaning ovens, then that's the stuff.
Lye reacts with most anything oily, fatty, or organic. (And it etches non-anodized aluminum!) **That includes your skin and lungs!!** When it reacts it creates a soapy film that makes it difficult to get rid of in an emergency, which is why it is one of the worst common household items to ingest. Most anything you try to use to counteract it just washes over that film while the lye continues to work underneath, converting your tissues to soap. It is a very effective cleaner for this kind of thing (assuming you're certain of the nature of the residue), but its hazards must be taken seriously. Lye is a base so an acid, like household vinegar, is an appropriate thing to have on hand to neutralize it if anything unpleasant happens despite your best efforts.
You probably already know all of this, but it's better to be overly cautious and instructive here than to leave anything to chance or assumptions.