I have to echo Sam's comments regarding Superfil. I started with Expancel
and West Systems Epoxy and while it is cheap, the effort of mixing the stuff
up and trying to get a good batch to batch consistency is just not worth it.
I also got a bunch more pins holes with Expancel.
The other problem I had was with moisture ingress. I had to repaint the
side of my aircraft where I had used it due to moisture finding its way in
and then finding its way back out again when it was baked after painting. I
didn't have any such problems with Superfil. Yep, its more expensive, but
in my experience it wasn't worth the effort.
Just my 2 cents worth.