You could also try jewellers rouge, if you can get it, mixed with water (and some soap or detergent for wetting). It should also work mixed with oil. The object of the exercise is to cover the surface you need not to take the solder with a powder which is not affected (dissolved or removed) by the flux.
I understand that in some trades whiting (fine chalk dust) is a useful alternative.
A topic for an article, perhaps – 'Solder Resists and how to use them'
Cheers, Tim