Hi Ben;

You're likely to get differing viewpoints on this, probably relating to differing cloth and sanding properties & techniques . . .

First of all, you can sand resin as soon as it's sandable - - Even when you are getting the little balls of resin instead of sanding powder, as long as the surface is not galling. I shot for that time frame to sand because it was so much easier to clean up.

As to sanding before fill coats, I never had a problem. On the hull, for instance, I laid up the cloth and two layers of reinforcement tape along the keel all at one time. Before any fill coats, I feathered the tape edges into the hull cloth and those areas remained perfectly clear. They are supposed to, that is what makes glass & epoxy so forgiving, you should be able to patch and sand and fill without leaving the slightest visible scar . . .

Be advised that the last time I said this, at least one poster said wherever he sanded his cloth left a sparkly-whitish appearance after more fill coats - Which I personally have never seen. I suspect it could be dull sandpaper, which simply burnished the sizing from the glass fibers, leaving the epoxy unable to wet the exposed fibers again. This is a guess, as I have never duplicated the problem here (Hey, I'm not complaining! )

Hope this helps, Spidey