Hi Mr Mayhem, I find it surprising you couldn't find a grain filler. I'm not sure how easy a resin/epoxy will clean up and sand flat. I would maybe try and spray some more of your sandable primer, maybe put on a few coats of that to see if it fills the grain, wet sand between coats and see if you can get it flat, then continue with blue coats.
I'd only try the epoxy/resin if the sanding primer doesn't work.