A filter is a massively thinned paint. You need to first apply a clearcoat to the model to prevent the timber absorbing the filter and possibly increasing the blotchiness.Testors dull coat in a rattle can is adequate.
You can buy filters intended for plastic modelling and airbrushing. AK interactive and others make these. You can make your own up but you have to be careful what paint you use as you need one with microscopic pigments to be able to thin enough. The white on my Soleil Soleil was done in this way. I thinned Vallejo premium airbrush paint using their thinner and flow improver.
You need to keep the airbrush a good six inches from the model applying the filter and apply lots of light coats moving quickly and not pausing in any one area.Allow each coat to flat off before there next so you can keep an eye on the colour build up. Stop immediately you are happy, further paint will start to obscure the features you want to show through.
It sounds harder than it is to do, my Soleil royal took 30mins to flash over the bottom including flatting off time between coats, the masking took longer!
I would seal the filter with another clear coat to prevent marring. Because it is so thin it has no durability.
I always did. It could be a little embarrassing, but always worth the trouble, not to mention repeat business.
