More pigment testing

While documenting the previous post, I suddenly remembered seeing someone use (diluted) white glue to fix pigments. Assuming white glue is a synonym for "wood glue", I did some more testing.

I diluted my white glue with water, then added pigment to the mix and spread it on my test card.
Bottom left : heavily diluted. Bottom right : slightly diluted.
In both tests, the pigments remain firmly fixed, but the right looks ugly and the left one is easier achieved with an actual wash.

Then I remembered something else. Instead of making a mixed fluid of glue and pigments, I made a new diluted mix of glue and water (but no pigments) and applied that to my test card. On top of that wet area, I applied dry pigments with an old brush, just stippling it on until the entire wet spot was covered.

When this had dried a little, I carefully touched the pigments. Half of them remained fixed, but many came loose. On a hunch, I added some more diluted glue all over the surface, letting it soak into the pigments.

Once this was dry, the pigments withstood even hard rubbing with a finger, while the looks are becoming more and more than of  thick layer of dust or dirt in a somewhat realistic fashion.
If I were to apply this to tank tracks, wheels or underside, I think I could make something rather nice.

The only issue remaining is how to do it realistically and not overdo the effect. If I use glue, and the result is too heavy, I cannot undo it. If I do not use glue, even mild handling of the model will shake the pigments loose. I was told white spirit would fix the pigments, but it clearly didn't. At least, not on my index card. I should test further on plastic with actual paint on it, to see if it behaves differently.
Maybe I can work out a technique where I apply the pigmenst dry to the model, temporarily fix them with white spirit and - when happy with the result - add diluted glue to fix it all in place.

More experiments will be needed and I'm a little hesitant to try it on an actual model.