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if the marks are caused by abrasion then you try polishing them out with some compound like this
its also worth try applying a little heat from a hot air gun to see if this helps as it will relieve the localised stress
yes I've used this product on plastic before with good results
a lot of plastics suffer and degrade with heat so i wouldn't say its anything to be concerned about
I'd check under the cover to ensure nothing is spraying fuel on its underside