I've used Durham's water-based wood putty, in equal parts with sanding dust from my hull sanding (50-grit), to fill some small gaps and dings in my hull, prior to 'glassing.

I let the filler dry for around a day, temperature is in the 70's.

Problem is when I sand the filled hull, much of the putty in the cracks pulls out of the recesses, it doesn't seem to want to "stick". When I looked through the archives, it seemed as if Durham's/wood flour mix is the consensus for the appropriate filler. So what's the secret for having it stay in the cracks?

Is 50-grit sanding dust a good approximation of wood flour?

(I note that some references suggest using microballoons in the mix, since I don't have any, I haven't tried this. Would microballoons add to the adhesion? I thought this was just a thickener to add body).

If anyone has any advice or thoughts I'd really appreciate it. Thanks in advance.