Don, I base my idea on a filter I once saw,,,,,,it had a mesh screen, with the charcoal in a layer, but, fluids could still pass through it,,,,,,,I figured a "large chunk" layer of charcoal, then a "large chunk" layer of rock , etc,,,,,,,if it will pass a machining fluid through at a relatively low psi, it should pass air,,,,,in fact, we used an air hose blowing through the boxes to clean them,,,,,,,we just unhooked the blue Fluid hose, squirted air through, caught the solids in a coffee filter looking strainer,,,,,,and hooked things back up
this thing caught paint, poisons, aluminum and gold flaking, and anodizing particles
it was efficient enough that once cleaned with the hose, it went back to duty, it should last a good long time with hobby paints,,,,,,,especially since we wouldn't care too much if we got paint flecks drying on the rocks
think of it as more of a "paint maze" rather than a bucket of sand,,,,,,,,the larger the "chunks" the better the air flows and the more places for the paint to "hit and fall out"