Here are some of our early results testing out wood filler from the local hardware store.(Remember to hover over to read the full description)
And some videos:
Also worth a look is Durham's Water Putty. It probably costs less and user can vary viscosity. But a concern may be working time. http://waterputty.com/
It's tough stuff and machinable. I've drilled & tapped cured water putty with surprisingly good results.
That prompts a thought: Will two-part materials, e.g. Epoxy, fit into your plans? Water Putty powder & water mixed just prior to the nozzle ... ??
I'm also trying to extrude Elmer's wood filler to familiarize with Discov3ry. Do you know the viscosity of the wood filler? Does it show similar viscosity as sugar frosting or Nutella?
Thanks! One extra detail on wood filler -- you might have to try a few to find one that works best. We bought a brand new container of Elmer's wood filler awhile back and could not get it to print at all. The stuff we did print was some old Elmer's wood filler I had in my basement. It could be that Elmer's changed the formula; or, perhaps the original stuff is still available and we just bought a new (improved!) version. So it's worth trying a few different kinds. You will likely need one of the larger tips for wood filler, too, since it has silica beads (like sand). Those will clog a small tip quickly.
The viscosity of wood filler is a bit more like clay than frosting or Nutella. One other thing you can try: most wood fillers say you can add water if the wood filler mixture is too thick. That can also help get the mixture suitable to print.