Just get on with sanding it. It’ll cost you next to nothing apart from your time. I reckon I could have sanded it in the time you’ve spent on here asking what is essentially the same question.
Three or four sheets of 120g wet and dry used wet with some soapy water will get rid of the old paint. Then get some primer and topcoat from a marine or boat supplier (Blake’s, International Toplac, Jotun are all decent brands) - put it on with a 100mm roller and then lightly brush it with a quality paint brush. Lightly sand the primer when dry before applying the top coat, using something like 180 or 240 g wet and dry. Use white primer of your top coat is light, or grey primer if the top coat is dark - that way you will get away with just a single top coat if you apply it properly - that’s what I’ve used for every grp boat I’ve ever painted.
Three or four sheets of 120g wet and dry used wet with some soapy water will get rid of the old paint. Then get some primer and topcoat from a marine or boat supplier (Blake’s, International Toplac, Jotun are all decent brands) - put it on with a 100mm roller and then lightly brush it with a quality paint brush. Lightly sand the primer when dry before applying the top coat, using something like 180 or 240 g wet and dry. Use white primer of your top coat is light, or grey primer if the top coat is dark - that way you will get away with just a single top coat if you apply it properly - that’s what I’ve used for every grp boat I’ve ever painted.