Last night, I had a nightmare that all my chisels had rusted over and bellied. So this morning, during my breakfast of mini-bagel and green tea, I made sure they had not. I actually only got through 1/4-5/8, and will do the rest tonight.
I love my 3×8 diamond plates for grinding and sharpening, but they are not ideal for lapping and flattening. When I need something perfectly flat, I turn instead to the machinist’s granite slab pictured above and adhesive-backed sandpaper of various coarseness.
For flattening chisels or plane irons, I typically use 80 grit, 120 grit and 220 grit sandpaper. I care more about flatness than mirror polish, so any additional polishing done on the diamond plates in the course of sharpening. For plane soles, I might go up to 320 grit (which is overkill for everything but a smoothing plane, I know).
I’ve experimented with sanding belts and spray adhesive in the past, but I’m never happy with the adhesion, and certainly loathe the mineral-spirits cleanup of the spray adhesive. With adhesive-backed sandpaper, I just need a scraper and a spritz of Simple Green cleaner. The grit on the sanding belts definitely lasts longer, but it’s a small price to pay.