There's a very cheap Chinese made chisel sharpening stone 8" x 2" x 1" I've had in the workshop for maybe 10 years now that I use for linishing gasket faces on heads and blocks and flatting cam followers etc. I think it cost only 1 or 2 quid when I bought it but I can't recall where I got it from. Maybe a travelling market that came to the village one weekend. It's very soft and friable which means it removes stock quite well as the surface turns into paste as you use it. I'd guess it acts about the same on steel as fresh 180 grit wet and dry paper on a surface plate.
I've also got a 'proper' one from an engineering supplier which cost several times as much which is fine for sharpening chisels but useless for my main purpose as it's too hard, doesn't break down at all and just skates over the surface. By contrast the Chinese one is useless for sharpening chisels because it's so soft they just gouge a groove in it.
As I recall it came in a cardboard box with blue Chinese characters on it but that's long since gone. The fine surface which is the one I use has now worn completely away and and I'm left with just the 1/2" thickness of the coarse side which doesn't do the job.
Does anyone know where one of these might be found as I'm kind of lost without one at the moment. Preferably near Slough so I can go and get it but I'd be happy to pay postage if anyone can find one.
-- Dave Baker