bulk up resin

[grp.boat]

what is the best additive to add to resin for bulking it up for sanding?

[fiberglass jack]

what is the resin going to be ( above or below water) micro balloons makes the easiest to sand but will sag , to stop this add 10 to 15% cabosil (fumed silaca) this will stiffin the paste add to much and it will be a bitch to sand