blast clean like you have, then wipe with acetone, then apply the jb (red and white original formula, not the 4 hr quick dry type, I find it (4hr) doesnt seem to like the bake cycle so much). Smooth it on there with a stiff card that will wrap around the pipe a bit, then smooth it on and let it sit, then sand like any other filler.
It will be ok for powder then.:thumbsup: