Having read giganticdark\'s Space Marine thread, I too wanted to have a try at crackling effects, so I got such a medium (not Vallejo or the one used by giganticdark, but I can\'t recall the brand for the moment) and did a series of tests on different surfaces, primers, and contrasts of colors.
As the Vallejo instructions, my medium said to apply one color, then the medium, and finally another color.
I started out with 3 light grey spots and 3 red spots, and waited 30 minutes for them to dry (this was actually done twice, once on white primer, and a second one on black primer, but it didn\'t change the end result).
I then applied the medium on 2 grey spots and 2 red spots, and waited 30 minutes as instructed (Note : the medium was a bit thick and gooey for my taste; I\'ll be doing additional tests with it being diluted). The first spots were for comparison sake.
I finally applied on the last grey spot some red, and on the last red spot some grey. And waited 5 minutes as instructed.
The last spots did crackle, but the paint looked thick.
I then went back to the 2 spots that only have medium on them. Transparent crackles had appeared. So I applied a wash of Devlun Mud to get giganticdark\'s effect.
It worked rather well on the light grey, though care is needed to avoid ringpools, but didn\'t look very convincing on the red (just messy).
My immediate though was that highlighting and shadowing with a brush isn\'t very practical for this unless you work on a light surface and use a wash to darken the cracks, or use an airbrush, or do it on the fly ... but results might be unsatisfactory.
For now, I don\'t feel confident with this medium and more testing is necessary

.