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Warren (Staff) - 8/12/2019 11:06:55 AM

It's not something that would really suit epoxy.  If its not visible, sand it all back, rust treat then coat with a chrome style rattle can paint so it doesn't look too bad if some one were to look.  A quick fix would be to apply Kurust or similar rust converters, then rattle can over surface as above.

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MarkMK - 8/12/2019 4:05:31 PM

I'm not certain that Warren picked-up on the fact that you're looking to take a mould from the bumper and thought, perhaps, that you were just looking to fix the rusty areas?

Rattle can paints are unlikely to give you a good surface from which to mould from and it would likely be attacked by the gel coat and come away during de-mould

You might just end up with some rough spots if you went ahead and moulded direct from it as is, but you might also get spots of gel coat sticking to the rusted areas that leave you with holes to repair afterwards in the mould 

If you're not minded to paint the whole thing with a reliable paint system in order to get a good surface all over, you could opt to cover the bad bits with flash tape if that element of the part would be mostly hidden when mounted (underside?). This would be the 'quick and dirty' option but probably depends upon how critical the affected area is to the part cosmetically. If you opt to use ordinary filler this would likely stick and pull away upon de-mould and the process of flatting it would also scratch-up the surrounding areas, so likely not a great option on its own.