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As for what Black to use, Alclad is the way to go... I tried Vallejo stuff and it was just too soft to use an heavy weathering techniques on...
Black as an undercoat for colour works brilliantly - you cant get more 'texture' and more control over the shade than you can a neutral or white primer... And as above, in my opinion, helps with panel lines too! You do end up working a bit slower (or maybe thats just me), but yeah, since I happened upon it a while back, I've not looked back!
D
Vallejo - the paint I like (although I tend to use mostly LifeColor Acrylics, ModelMaster Enamels and Alclad Lacquers), but I tried the Surface Primer (in Black) a while ago and it was TERRIBLE! Firstly, if you prime and find anything that needs even a LIGHT sand, forget it! As soon as you sand/polish the Vallejo Primer, it just tears off the surface. As for weathering (happened on my D-11 the other month), while working with crushed pastels, to create exhaust staining, I guess I got a bit 'vigourous', because the combination of Crushed Pastel (abrasive) and presure (from a short bristled, stiff brush), once again TORE the Primer off the plastic!
The Alclad on the other hand, being a lacquer, actually forms a chemical bond with the plastic surface, dries MUCH harder and sands/polishes much better. My technique usually involves lightly polishing the primer before colour is applied; something thats impossible with the Vallejo Primer.
As for WHY I use Black as undercoat/primer, I THINK we agree on this - not only is it a much better primer colour for Alclad Metal Lacquers, but you get much nicer colour (more 'depth' or 'texture' or 'feel' or something). Also, I have noticed that if you apply light coats of colour on black and keep the air brush at an angle to the surface, you get less colour paint in the recessed panel lines. This gives them a slightly darker appearance, even before applying a wash.
I hope that translate better for you?
I'm Australian - the rest of the English-speaking world has trouble understanding us most of the time anyway. Makes sense that Google-Translate is the same
Dan