Yes, you can substitute the wood flour mix for silica for glueing the boat up. The silica will dry a milky white. Cell-o-fill is not silica, it's a cellulose product that's not harmful, unlike silica. I use silica to glue up panels (scarf or puzzle joints) and structural items like bulkheads, and also daggerboards, thwarts, rudders, etc. I use wood flour for everything else. I have a buddy who uses nothing but wood flour for all phases of construction. CLC sells Cab-o-sil which is the MAS version of colloidal silica. 
 

George K

   1 Wood floor thickened epoxy is often pretty coarse and lumpy. The syringe tip will need to be cut biggish, but the stuff will still work.

2 Fumed silica mixed into epoxy is a translucent milky white. It is the hardest, and brittlest, and toughest to sand mixture we use. It's appropriate as a fillet under fiberglass tape, but I'd use a more fiber-based filler for untaped fillets- I've seen silica-based fillets crack on their own. I often mix a little silica into a wood flour fillet blend, since the wood is usually too dark on its own.

3 Cell-o-fill is cellulose pulp/powder, not silica at all. It's probably closest to wood flour in properties, but smoother and white. It's lighter in weight and much easier to sand than silica.

Searching for "fumed silica" should get more results. I've bought it by the trash bag full from companies like Raka Composites or US Composites. It's a commodity, so avoid buying it from boutique sellers...

���Thanks, I think I'll go with the wood flour route. The manual shows them filling in some spaces from the top side, think I want something that sands easier up there. Cheers