What STAINABLE wood fill is best to use on a plywood floor?
There is no good filler for every purpose all wood and all fillers take stain differently. When they say "sands and stains like wood" they are misleading you. Different sections of the same tree stain differently. That's why quarter sawn lumber is desirable. You need to try a swatch in a descret location. Or better on a sample. Finding something to match plywood may be a bit tricky. I'd avoid putty. Try to get a filler of the matches the species of wood, if you are able to identify it. Another method is to make your own filler out of fine sawdust of the same material, if you have a peice available. You make a thick paste of the sawdust and wood glue and apply it as filler. It's very hard to get a seamless finish and the colour of the stain will make a big difference as to how noticeable it is.