I am sure there are a thousand ways to do this very same thing but I just wanted to post how I go about it.
After I sanded down the pink (turned white when ready to sand) filler, I then layed down a thick coat of white
latex primer. Once that was dry overnight, I then over the course of 3 nights layed down 3 coats of epoxy in which I had added a talc filler into. This built up a nice thick layer of epoxy. Fairly un-even as you might imagine using a 2" brush to apply it.
Next, I sprayed a coat of automotive primer to the entire hull.
Once that was dry, out comes the long T bar sanding block and 80 grit paper.
I start on this radially with a forward-rearward motion to avoid getting any flat spots.
As you can see from the picture below, as the Grey primer begins to sand away it leaves a really nice contrast
to the white primer. In this way it is very easy to see exactly where the low spots are and as you work them with a large overlapping sanding stroke they slowly start blending away. The talc in the epoxy also makes it much easier to sand.
I left a portion half way done just to show the progression.
I have used this high contrasting approach before on airplane fuselage work and it pretty much leaves a perfect finish the first time through. It will take me a 3 or so more days to work my way down the hull (no need to rush things) to get the entire hull to this sanded state before the next steps.