After installing the core,  the next job is to apply a prime coat of the resin and then you should go over the whole hull checking the fairness and applying resin putty or fairing out any imperfections, where necessary.   It is important when building a fiberglass boat on a male mould to see that it is as perfect as possible before going on to the next stage.   Different core materials will absorb different amounts of resin prime coat.   With balsa core, after filling any gaps with resin putty, you may want to apply one or two thinned coats to seal the surface and then another coat of regular laminating the resin – check your suppliers recommendation    PVC core will needs at least one coat of resin primer.