Flattening India stone on a cheap granite surface plate

UPDATE: I tried flattening my India stone on the back of my granite surface plate using wetted 80 grit silicon carbide powder. The surface plate is 12" long and 9" wide. The stone was flatter afterwards. However, subsequent attempts with my Crystolon stone revealed that the underside of the surface plate was becoming increasingly dished. Consequently, the surface of the Crystolon stone started to become slightly convex. I've since obtained a piece of glass 21" long, 14" wide, and 1/4" thick for lapping my stones on. I continue to use wetted 80 grit silicon carbide powder. The difference is amazing. I think the larger surface area of the glass increases the effectiveness of the lapping process. I've used the glass several times for this purpose and it remains flat. My stones are flat to the point that I can't insert a piece of very thin greaseproof/baking paper between a stone and straight edge that doesn't deviate from true straight by more than 0.001" per foot, at any point along the length or across the width. Good enough even for fussy me!
 
80 grit should be fine for leveling a India stone. It should not have become glazed. This is likely the last time your stone will need leveling. DM
 
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