I did one kayak with a quart of silica... plenty for the places I needed it... I still have about half of the quart left. I did not use any microbaloons. If you need additional fillers, you can use flour or sawdust. Both work pretty well and are cheap.

When you use the silca use a good quality filtered mask. The stuff gets into the air and into your lungs very easily. It is NOT good for the lungs. Once it is wet into the epoxy, it is much safer again. It is Wonderful for forming a thick slurry which does not sag. Sawdust is good for the same reason, but it is dark brown.

When you get your can of silica, you will probably do a double-take and think they sent you an empty can... but it will be full. The stuff is nearly weightless.

Rick