Any of the above, and if you find stubborn spots, 0000, 000, 00 steel wool will rip it off but not roughen the mold finish or remove metal measurably. (If you quit scrubbing when the dirt is gone.) I named the grades finest first, which is the order of preference. 00 will be quite agressive and can scratch if you press real hard and rub too frisky. HOWEVER, USE SOLVENTS AND SOAP ONLY IF YOU WANT TO CLEAN THE OUTSIDE! KEEP IT OUT OF THE CAVITIES! MOST LEAVE A RESIDUE WHICH SMOKES OFF ONLY AT VERY HIGH TEMPERATURE, AND CAUSE CASTING PROBLEMS TILL THE RESIDUE IS GONE.
If we are talking about new blocks, don't clean them. Just start casting. The only 'contaminant' on them is bear lard, which is the only cutting oil I use, or chicken fat if bear fat isn't available. Both smoke off by the time the mold is up to casting temperature.
If we are talking about mold lube stain on used molds and you aren't going to show them, and there is no buildup holding the blocks open or filling the cavities, don't bother to clean them, ever. (Even if returning for my attention, I won't be embarassed!) A film or patina on the surfaces is an asset, as it helps the cavities fill better, because it's a slight insulation between lead and aluminum. Also, any LBT mold lube coating will prevent corrosion should the blocks get damp during storage, whereas chemically clean aluminum will corrode with a passion in the slightest dampness, which will ruin the blocks if corrosion is severe enough to change dimensions.
Some years ago a customer returned a 4 cavity mold to me, with great pride, and asked me to check it out. - It was clean and bright as new, and still cast within .0002 of round after casting 400,000 bullets! .0002 is slightly less than half the thickness of saran wrap! - My point. This man was fussy about how he used the mold and didn't beat it up when casting. Every time he put it up he scrubbed it clean with a green scotch bright pad. No solvent. So it still looked new after years of service. But clean had nothing to do with the long life. That was due to carefull handling only. So add scotch bright pads to the above list.
