I've built several small boats in the last few years and often used sawdust to make cosmetic filler to hide my many mistakes. It worked fine. I made the saw dust by clamping my belt sander in a vice and holding a scrap of the wood I was trying to match against the belt. What is your application?
I keep several jars of sawdust from various species for color match touch up, both with epoxy and Titebond III. They work great. Have also used corn starch in a pinch (when the First Mate wasn't looking !
I've also used sawdust for wood repairs on the interior (fixing some water damaged teak ply). For everything else, I usually buy the commercial stuffn (mostly fumed silica). Most of the fillers can be bought for less than West prices.
Regardless of what filler powder you mix with West epoxy, the cured result will be as hard as diamonds. Try to do the majority of your shaping while the stuff is still a paste.
As Mike said above, hard will be. There's no way water will disturb the wood powder inside of the fillings if it is pre-mixed before application.
Yet, if you still want to go alternative, try industrial talc powder.
I have used sawdust many times as well. The dust from 100 grit paper on an edger (floor sanding machine) is as fine as flour. I keep oak and pine on hand in a plastic coffee can with a snap on lid.
Mayby a local floor sander would save some for you. We produce a lot of it over a year!
when adding thickeners to epoxies keep in mind that some brands of epoxies are way more brittle than others. Those with very very brittle epoxies are the least likely to mention brittleness issues. Bittle epoxy is not good in dealing with impact issues.
of course when using epoxy with fiberglass cloth, all the issues and properties change.
thanks guys. guess I will save some bucks and use sawdust. The damage is between the deck and ceiling of the upper deck - the corners and right accross the stern. all repairable and it will be better than new.The flying bridge brow board and the teak deck had been leaking for years. Had to remove the old teak deck also but will salvage some of that to repair the main cabin floor next year. will do a new fiberglass deck upstairs after repairs are done.