[Arran]

Does the CrystaLac small like acetone or lacquer thinner by any chance? That may explain why its so difficult to work with. In any event since the Constantine's filler works fine there's not much point in messing with something that's more frustrating to work with.
I only wish I could find proper grain filler where I am, I checked around at some paint stores a few years ago and came up empty. I have been experimenting with something called Elmer's Wood Putty, it's water based and has to be thinned out considerably to fill grain, but you can sand it. I was trying on a battery set cabinet and it seemed to work well, the cabinet was already sealed by the way, I have not tried it on raw wood yet. At one time they also used tinted plaster for grain filler so it's not that outlandish of an idea. One good thing is at least there is a local dealer that carries Mohawk products, though I'm not sure about the selection at present.
Regards
Arran