If you have applied any bog to a car you will know that it's impossible to get the surface spot on first go.

The same applies here, you use a long sanding board and sand until the high spots of the keel jus start to appear. Do this to the entire length that has been faired. You will notice that you still high and low spots. In some cases I will hit glass and still have low spots. At least now the low spots won't be as low as when I started. Repeat the process, but this time I won't need to apply as much. If Iam lucky it should be just how the plans say after two goes. If Iam really fussy, I might spray a very fine mist of black paint over the fairing and lightly sand, any paint that disappears is a high spot, so I either sand some more or fill the low spots. It will depend weather the high spot is glass, if so I will have no option but to do some more filling.

I weighed the port hull yesterday and it came in at 22.8 kilograms. So with a bit more fairing to go, a little glass around the bow , the deck to be added and then the paint, Iam hopeful it will weigh in at 26 kilograms.
This photo shows pallet strapping holding the hull to a saw horse while I work on it.