Fibre glassing question..Fabby ??

[dogwood]

Hi.
Im about to modify my Triton scuttle. (Widen it)
I have made up the "Splash" molding as Mark sudgested.
Question is,
Do I need to use Gelcoat before the matting or will just the resin be OK?
I intend to spray it anyway so finish is not an issue.
I have some very old gelcoat in my garage.(25 years )
Does it go off?
Sorry if these are dumb questions, but what I know about fibreglassing could be put on a postage stamp, and still have room for the queens head....

David
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