Right easy one to Ans
Yes it dries hard this is what we use for body repairs so it has to be
VERY easy to sand..........Far BETTER than P38
As for Being Waterproof............Er NO
Right let me put a few MYTHS right here with reagrds to Car Body filler...
No car body filler at all is Waterproof, NONE AT ALL....as a Matter of fact it is Porus if anything, and CAN if left unprotected let in water....I must confess to having a big chickle to myself whan i see car body filler being used on the INSIDE of a hull to fix in Prop tubes and the like.It WILL eventually crack and let go......You should ALLWAYS use Isopon P40 or equivellent GRP FIBER PASTE.
It is the paint that protects and Waterproofs ALL Fillers.
TBH with you Primer by it's nature is also porus more so than filler ...thats why I advocate DRY sanding all the time to stop water ingress into it,let alone DRY sanding of the filler.
Dave