300-400 sq feet per gallon coverage with kilz primer
250-400 sq feet per gallon coverage with your behr exterior paint.
With such a good primer base I would expect 325-350 sq feet per gallon on each of your 2 finish coats.
I use a shellac based primer for things such as this. It has better hiding ability then just paint or regular primer.
Give it at least two coats or more and see if it hides the problem and then you can apply the final paint. It goes on very thin and appears to not be hiding the stain but don’t let that fool you it is creating a impenetrable barrier over the ...
You may have wondered with all these low odor paints available why would anyone use a smelly paint? One answer is to find a paint that will not absorb the stain.
You don't need to sand back to wood, just scuff sand to dull the sheen so that a fresh coat of different paint will stick. however if there is a texture you want to disguise you can sand deeper.