Designer's gouache from any art store...
David Barnett - SW Florida
>...can be thinned with water, rolled*, daubed, sprayed or brushed to leave a fine opaque matte surface. Easily washed off, if you prefer you can 'fix' it with ordinary hairspray or art fixitives. Acrylics dry quickly but gum up files. You can also use sumi ink, but the amount of binder varies in these. Don't bother with the more expensive 'artist' gouache, btw, as they're formulated for permanence and historical correctness. I often use gouache as a ground for engraving layout. I prefer the velvety matte surface to the shinier and rather too transparent Dykem unless I'm marking by scratching or cutting through the dye film. China white, sold in little cakes, works okay, I suppose, but can be a little crumbly and chippy. Besides, I prefer a grey, buff, or grey-green surface. Easier on the eyes, IMO.
Of course, Dykem or Starrett layout blue works fine. The Starrett spray is convenient but pricey. Make your own from dye, alcohol and shellac. Dykem makes a wide range of metal marking dye pens, too, but Sharpies work well enough. Most layout pens are alcohol soluble.
*I'd think a little bench-mounted reservior with a 1/2" wide roller in the top would work great for anyone who sharpens lots of saws.