[MrNomad]

In one word, my experience with CAB-O-SIL was excellent. The cowl for our RV6A was assembled by a prior builder. The problem we inherited was that the horizontal hinges didn't match so the upper cowl protruded over the lower.

Our technical counselor offered two ideas. Put a spacer behind the lower hinge so that the lower cowl matched the upper, or build up the surface of the lower cowl with WEST and overlapping, blended layers of cloth.

I chose the latter method. After D/A sanding the two layers of cloth, I mixed CAB-O-SIL into WEST. I didn't make it as thick as peanut butter, but close.

Next morning, I "D/A ed" the CAB-O-SIL and I'm pleased to report that I achieved show quality smoothness on Chet's 6A cowl.

My thanks to Gil and Dan Horton for their helpful suggestions.

 

[DanH]

You're welcome Barry, but perhaps a comment is in order so as to not create confusion.

I assume you selected cabo as a fill additive for a special reason; you wanted to ensure the epoxy coat would not run off a vertical surface. When added to mixed epoxy the resulting fluid only flows under pressure, much like toothpaste. The classic use is forming a small epoxy fillet in a right-angle joint with a fingertip, like when you glue wood ribs to a spar. You can wipe it in, it stays there until cured, and it doesn't change epoxy strength very much.

However, cab-o-sil (trade name of Cabot Corp, synthetic amorphous silicon dioxide, SiO2) isn't a good bulk surface filler choice for our airplane application. Sure, it sands out ok, but it's very heavy compared to glass microspheres. The original Rutan callout for micro was 3M 1323 500 psi glass spheres. Today I think the equivalent is 3M K20, which weighs 0.20 grams per cubic centimeter. Cabo weighs 2.2 g/cm^3, more than 10 times heavier than micro.