I found some old threads; seems for a time the freshly welded steel went straight to the powdercoaters, and they did NO prep, just shot & baked.
I have a bit of flaking on my engine mount also, probably a circa 2003 or so.
I chipped and shaved down with a wire wheel, did a close visual inspection, primed and touched up as needed.
Not a permanent fix, but your mount isn't exactly 'out in the weather' so future corrosion seems unlikely.
RV-9a - SOLD!....
Lake Country, BC