Be aware hammerite doesn’t fully cure by its nature so you might ‘smear’ the hammerite. Someone I used to know decided to paint a Fiat X1/9 with hammerite and thought the finish was a bit dull so was going to lightly rub it down and then polish. Didn’t get to the polishing as the hammerite smeared everywhereI have done exactly that. A few coats of smooth Hammerite. Would be a lie to say that both caps look as good as new, but certainly looking much better than they were. Modern Hammerite does seem to retain brush marks much more than it used to. Might go over it with some very fine wet and dry. Considerably less pricy than 2x new caps. Also less risk of breaking screws and then not being able to fit replacement caps.