How to paint window ledge over heater?

I've scraped away a small portion of the paint off the metal top over my heater and see rusted metal. There's some kind of coating between the metal and paint - is that spackle or plaster? To make everything look new I think I have to gut everything, replaster and repaint. Is this right?
I'm in a NYC prewar apartment with a recessed heater under the window.
Thanks.
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Hard to see from here , my eyes are bad, Id say Strip with chemicals or easier use Real spackle. Or Bondo. Bondo takes a bit of experiance good luck
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colors of paint. I'd remove the parts you can, and send them out to be stripped and sprayed with a heatproof paint. If there is a metal box mudded into the wall that the cover clips or screws into, part of the mounting edge may be behind plaster. If the buried parts of the metal have rust, it sounds like you have a water leakage problem from radiator and/or window, or a lot of condensation. If the plaster around the window is soggy, or has been, there is no good repair short of removing all the damaged material and building back out to finish surface. If you just patch and paint over it, it will keep coming back. Gotta stop the leak.
You own this unit, or is it a rental? If a rental, this is the type of stuff landlord is supposed to fix. Sounds a little ambitious for a DIY job, especially if the actual radiator has to be pulled to get access to clean it all out. If you own the unit, the super or management company will probably have to get involved if there is a common boiler. They will also probably have the name of the guy they usually use. And on a radiator that old, expect that parts will need replacing, which can be much fun to track down. May be less work/cost to rip it out, patch the hole in the wall, and put in a modern radiator, assuming a compatible one exists. aem sends....
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