Hi Slarti, gel-coat makes fibreglass waterproof - without it water can get through the paint and discolour the surface beneath.
Assuming it's a female mould, I'd give the inside a dozen coats of release wax, each buffed to a shine and left for an hour before the next, then a coat of catalysed gel-coat, followed by four or five layers of CSM once it's tacky to the touch. That should give you something that's heavy duty and weatherproof.
Good luck and welcome back.
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