Microballoons are not stuctural. Let's compare them with Cabosil. Under a microscope, cabosil granules look like little spur rowels, or snowflakes, with spikes and angles all over. When encapsulated with epoxy, these spikes interlock with each other, creating a strong matrix. The tensile strength of cabosil mix is greater than neat epoxy, although the compressive strength is the same.

Microballoons are little hollow spheres, so they do not interlock. They come in two flavors, quartz or phenolic, but the difference is minor. Their purpose is to bulk up the mix with a lightweight filler that is easy to sand. By the same token, they are not thixotropic either, so if you're working on a vertical the mix will sag. No amount of additional microballoons will prevent sagging. Put in some cabosil though, and it will stay put.

Wood flour, or dust from your sander, is both a light-weight, easily sanded bulking agent and a thixotrope. It's also free. So if microballoons are hard to come by, sanding dust works fine and does not sacrifice any physical characteristics to microballoons.

End pours are not structural, so you can throw in wood scraps, foam chunks, or other light weight trash to bulk up the mix.

: I'm building my first kayak this winter,a Pax 18 from CLC and have the hull
: assembled and glassed inside.I have been having difficulty finding
: microballoons
: to thicken the epoxy for the bulkhead fillets and endpours so I went to a
: local fibreglass supply shop and they didn't have microballoons but had a
: white powder
: called Q-cell powder which the salesman said were the same thing.
: Is anyone familiar with this product,my concern is that it has the same
: strength to weight ratio as microballoons.
: I would be greatful for any information .
: Thanks Bert