As opposed to chipping mediums, I've found that hairspray applied with an airbrush works very well.
I apply my preferred undercoat, usually aluminium, wait a couple of hours or even a day and then apply the hairspray over the top. I wait another day and then apply the camo.
I use Tamiya acrylics so after a couple of hours I can get to work with the chipping. All I do is brush the area for chipping with water and wait a minute and then use a cocktail stick.
I first used this technique with my Ju87 and was very happy with it. For large areas with high traffic areas like wing roots and the suchlike I just use a sponge with XF-16 to create a worn effect.
https://flic.kr/p/2k6jAif