Depends on what type of whetstone, and what you want to sharpen, but
just water is often used - perhaps with the addition of a small amount
of vinegar. If you do use oil it should be something like a light engine
oil - vegetable oil will also be ok.
If it's a rotary whetstone used for 'rough' sharpening - water will be OK.
If it's an oil stone used for the final sharpening of chisels, plane irons
etc - then the old 3 in 1 oil is ideal for this, or any other light oil.
I largely agree with you, Brian, but I didn't think that the OP was
thinking of a chisel sharpening oil stone. In an earlier thread he
referred to having three whetstones - " 2 rectangular(1 white, 1 grey)
ones and a boat shaped one". It sounds to me like he has 'natural stone'
whetstones for honing scythes or knives. These can be a very fine
'grit', and not just for 'rough sharpening'. The stone is lubricated to
clean the pores and prevent it from clogging, and for this purpose it is
not critical whether water or a fine oil is used.
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