MIG Productions filters have become my one of my favourite products, for enhancing the appearance of model vehicles, and I've only used them so far on two different tanks.
I purchased the Allied and German filter sets after reading several reviews on the 'net', all of which spoke highly of this product. So far I have only used the Allied filter set. I got mine from Antenocitis Workshop. They are not cheap at £15.99 a set, in which you get three bottles. But I think worth the investment.
The filter should be applied after the initial base coat(s) have been applied and 'is used to enrich, unify and cause variations in the tone of a base colour'. And that's basically what they do. I just followed the instructions supplied in the packaging, and away I went. On the M5A1 I used just the BROWN filter for dark green and applied three coats. After the first coat I didn't notice any appreciable difference, but by the third coat you can see the filter taking effect, altering the tone of the paint. On the M4 Sherman I applied two coats of GREEN for light green and then one of BROWN, to slightly darken the overall effect.
The instructions recommend, 3 to 5 coats of the filter and allow at least two hours drying time between coats (it's worth it!). One aspect I struggled with initially with the filter is the idea it is not a wash.The filter should be applied with a damp brush, do not flood the model. Now on the Sherman I experimented with applying the GREY filter for bright green, on select areas, but I loaded the brush to much which resulted in 'pooling' and this looked messy when dry. I was able to re-work this, but was glad it was only on a small area. So far on each model I have coated the whole model (except tracks of course), but will experiment no doubt on future projects but adding successive coats to specific areas to see what effect this has on the appearance of the model. For example, using a couple of coats of GREEN filter overall, and then using the BROWN filter on parts of the tank, were the paint fading may not be so distinct (areas under shade).
These products are aimed at the 1/35th scale model tank society, but can obviously be applied to any scale. For further articles which can provide help, advice and inspiration look to Missing-Lynx and Armorama. Looking forward to trying out the German set. (I wonder how the German BROWN for dark yellow would work on an Allied green.....?).