Hey,
I think that Japanese guys are experts in this technique. Also MIG builds outstanding models using this method. I 've been using them for several years. And I'm happy with results, too :-)
IMHO this is the only way to reproduce real looking weathering (this is the BIGGEST pro). I use them for dry & wet applying.
I remember- in the past I had problems with applying them because: I choiced wrong colour or I didn't prepare surface for them properly
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To avoid this you have to study photos first (after using them for some time you will know what colour is suitable for what region) and to apply dry form only on matt surfaces.
Michal
P.S.
After September, 2002 MIG's company will release special set of pigments powders for weathering