Not on a chassis but my domestic radiator. A piece approx 40 x 30 mm has peeled off. Rad is traditional cast type ( not panel ). I am not going to remove it or strip it down I just want to do a patch repair. Any ideas? TIA

I've always used nail varnish for touching up powder coat scratches and chips, but never on such a big area - might be worth a try?

Not dne it myself.

I'd check round the edges for any more powder coat likely to peel away.
Mask the area with painters tape and newspaper.
Gently sand back with fine sandpaper to clean the area and givee a key.
Use an aerosol filler primer.
Use an aerosol top coat that matches the rest of the radiator.
Run the heating for 1/2 hr at a low temp to cure the paint.

Like I say not done it myself so no guarantee it'll work.

Fine sandpaper to feather edge the paint outside the chip in can understand. Wouldn't you want quite a rough sandpaper to key the metal?