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.
Filler I allways use either Top Stop or Easy light...................Main reason they are so easy to rub down and get a pin hole free finish.
David's P38 and such like I DO NOT ADVOCATE for boat work as it is so difficult to rub down .
The fillers I reccomend are good for wood,Plastic,Metal.
there are others such as Squadron putty etc but I DO not advocate them as they have a tendancy to shrink and have caused paint problems to more than one modeler on here.
For fixing in Motors,rudders,Bow thrusters deck beams etc you need p40 grp filler paste and NOT car Body filler as this could crack .
Dave