I have use diluted muriatic acid on concrete and gel coat.
However, I do not know the result if you use it on emron finish. BTW is
this finish also known as DuPontT Imron® Polyurethane Enamel? The best
thing would be to experiment with diluted muriatic acid before using it.
The last time I have use diluted muriatic acid was to remove barnacles. The
timing was crucial. Soon as you are finish removing the rust or barnacles
you have to rinse the surface immediately with lots of water. Failing to
immediately rinse the surface with lots of water could deteriorate the
surface finish and fibreglass.

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I used to store my boat near a RR trestle and every spring would have
to remove freckles from the deck caused by minute steel molecules. We
all used dilute muriatic acid very carefully.