I've used matt black cellulose brushed straight on the underside of a GRP bonnet for 9 years and it's lasted ok. It depends if you want a 'coating' or a 'cosmetic coating'. A useful thing (for underwing areas likely to get stone hits) is the super-elastic not bitumen coatings for waterproofing flat roofs. I can't remember offhand the brand name, but it's about £30 a can (2l?)
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