Neil,the only wat to remedy "cracked" fibreglass is to grind out,down to waiver thin,at the actual crack in a graduated "VEE"aprox 3to4 inches either side of it,you may need to "brace the work piece with a simple pop riveted piece of ally or simular depending on the size of the crack,as a temporary support,you should pre cut several strips off glass matting,get an old dish or hubcap,mix your resin with a drop only of catylist,immerse your strips into the resin and stipple in thoughly with a paint brush,when totally absorbed ,transfer the strips one at a time to the workpiece,(its at this point you will realise you need rubber gloves)and stipple in each individual piece,using the mixed resin,leave to thoughly cure,lightly sand ,you need to inspect it carefully at this stage weather it requires more glass/resin, or straight on to filler in the normal way prior to primer,This may read as clear as mud,so if you need to call me on 07802962991,bry