Microballoons are not intended for structural fillets. They will make the fillet weaker when used in any amount. They are used in fillets which are more cosmetic than structural.

However, any fillet will make the end result stronger than the same structure would be without fillets since bonding surface area is increased and the fillet creates a brace.

The glass or phenolic glass spheres make the end result lighter weight (which means you don't have to use near as much epoxy), and easier to sand. That's the only reason they are used at all. The phenolic balloons are easier to sand and probably more structural and have a dark color.

You add them to your epoxy after you have completely mixed together the resin and hardener in the proper ratios. How much you add depends on what you are trying to do. Add the glass spheres until you get a consistancy that is usually compared to a food product. Syrup, Catsup, Mayonnaise, Peanut Butter, Add more micro-balloons still and you end up with different densities of hard foam.

If you sand or drill through epoxy with glass microspheres (which are hollow), you will want to give it a light coat of epoxy to fill all those broken spheres on the surface.

You use milled or chopped fibers or wood dust (dust from sawing wood is better than from sanding wood) in your fillets to make structural fillets.

I used wood dust with a small amount of milled glass fibers to make structural fillets between the deck and coaming on my North Star. After forming the fillet and while it was still wet, I added 4 OZ cloth cut on a bias from deck surface and around the coaming. The cloth helps to smooth the surface and makes it alot more structural and was needed over the coaming. The wood dust made the color I wanted, and is a thixotropic (thickening) agent, in addition to making a structural fillet. The milled glass fibers made it more structural and was added since the wood dust fibers are very short. Milled glass fibers are about 1/4 inch long.

Visit some of the epoxy manufacturer's web sites, you can get some good info.

Hope this helps,

Doug K.

: Does using microballons in the endpour and/or for fillet work make the end
: result stronger, weaker or just lighter?

: How much is too much?

: Thanks for your time, M.