Both the online paint supplier and a local guy i used for paint sundries advised against etch primer on fiberglass with celly paint, what they did advise was to use a primer used on plastic bumpers to give a good key and help to seal the fiberglass surface which was put on first before any paint work commenced.
This is what i used.
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The spray finished surface has a metallic satin look.
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I do know there are guys on here who have used etch primer and are not reporting any issues as of yet so mixed messages possibly but my gut feeling was to follow the advice from people who work with paint.