Fix It All is the neatest stuff.
I have a section of bathroom floor (it was the kid's bathroom) that had some dry rot. After getting some guidance from a contractor, I've just about got it all fixed up. Chiseling out the dry rot was easy, no need for a mallet, just a firm hand on the chisel and the rotten would would come out! It would fall apart like a crumbly oatmeal cookie.
But then, I needed to fill all the wood I removed.
Fix It All is the answer. It is a powder that you mix into a paste and then put into the gaps like plaster. It is sandable, drillable, sawable and acts pretty much like wood.
In order to do this, I even had to get a new tool - a trowel! I always say, "New project equals new tool!"
I'll try to add some pictures later.