mold lay-up problems need help

[bbh57]

I've made a mold for the rear cap on my boat. I thought if I used gelcoat first in the mold I would have a finish without pinholes that would be eaiser to fair before paint. I put a few coats of gelcoat on and allowed it to dry then glassed a layer of 3-4 oz mat. when it kicked it melted the gelcoat and made bubbles everywere. what did I do wrong