Hi Brian,
Is that Fumed Silica? It is very fine, extremely fluffy material. But I don't think it's a good filler that sands easily. I don't think it will do the job on the finish that you are looking for but I don't know for certain. I would test first.
I don't use microballoons in the finish either. Much too coarse and needs extra filling so paint can stick without pinholes or bubbles. I don't recommend it as a top coat. Great underlayment tho.
So that we are not accused of highjacking this thread, maybe we should this take this discussion to your other thread?? What do you think?