Hand Tools Archive
google and ebay will find you the best price on the formax/microfine compound.
From what I can tell under the microscope, it's more coarse than 0.5 micron by a long shot, but it's a little bit irregular in grading for the alumina (which isn't a bad thing). Point being, you can go two directions with honing compounds - very fine (true 0.5 micron chrome ox, or something like linde b alumina -which is 0.3 micron, but will be more expensive - or more coarse, like a 3 micron alumina.
I'd stick with the green stuff after 3 micron diamonds - it's a good fit.
Autosol paste is another option (from smell and feel, the tormek PA-70 paste is just autosol, too, but at a higher price) - it's supposedly something like 3 micron, but it's a bit faster than the green stuff and the abrasive is a less tough type of alumina. That would also follow 3 micron diamond well, but no need to go there if you like the green stuff.
There's a million green chrome sticks from something that's $1.99 in the clearance aisle at sears (will be a harder stick and coarser alumina) to truly pure green stuff sold for razors in wax only with no alumina - but the comments above hold - the combination in the microfine stick is good for practical use because it's not as fine as pure chrome ox, but not nearly as slow because of it.