If it is the valve seat(HP orifice) then there is not much you can do about it unless it is the FSR(1st stage).
Anyone know a good method to polish it?
I would probably just replace the housing. I am the reapir tech for my shop (just haven't seen this issue before,... & it had to happen on my regulator) & I can get it ordered. Yes,... will probably be some $$$.