what do i do after the 6 coats of fb

if you rabbit joint the back side of the box correctly..and pull the fleece tightly, and staple it down well..you should have trimmed it nice and tight to the staples after your firt coat of resin was applied and cured...the rabbit joint would have allowed for additional layers of glass to be built up with no loose pieces sticking out..

your fiberglass should be sanded between every coat, if you allowed it to cure inbetween..if you laid up six layers of the hop..your crazy...lol...just kidding but, 6 at once is too much...the heat generated from that curing could cause a fire...not to mention the inner layers may not cure properly, as the outer ones will tend to cure slightly quicker...any out gassing could be trapped and creat air bubles in your piece...not to mention they could be large enough to deform the piece too..

at any rate...once your are happy with over stiffness of the box VIA thumb testing/ standig on it..your could either use a short or long strand fiberglass product to fill in any low spots, or you can go straight to filler...generally if you have laid the glass up on the outside of the box, your gonna want to kitty hair it first then use filler after that is sanded smooth..if your built up the glass on the inside of the box then the surface of the hardened fleece should be suitable to accept a smooth layer of filler after it has been sanded...

if your project's shape will allow it i would strongly recomend using a block sander for 95% of the sanding...