Don Foreman writes:

Not alcohol, you need a fast-drying, non-polar solvent.

Why is a non-polar solvent preferred?

I assume the marking compound base is non-polar: linseed-oil-based prussian
blue, my shopmade iron-oxide-in-light-machine-oil.

As someone else pointed out, there are water-based marking compounds, which
would be solved by water or ethanol.

I prefer the machine oil marking compound, since it doesn't dry out like
water-based or artist paints. When you have a sub-thousandth layer, it
dries quickly.

A brayer is OK to spread it, but your fingertips are more accurate and
even, if you aren't squeamish about clean-up.