Alright Dan.
When I've done facing work in the past I use a piece of plate glass with at the most 600 grit emery paper on. Plenty of water and a light action. You should be able to see any low spots show up quickly under a light.
I don't use any gasket sealer, rather a light smear of CL grease on each face and then the gasket.
I hope you get it sorted. A file? Eek.