Maxwell Render

Say I have glossy rough plastic chipping away to reveal aluminum.

I need how many weight maps? Three?

One 80% white with black holes for the rough plastic
One 20% white with black holes for the clear glossy part

...and one black with white holes for the aluminum?

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Do I understand correctly? That's a lot of memory for 3 nearly identical maps.

you need only two, because the 80% is just the 20% inverted...

Material A has two layers, an 80 and a 20

Material B has one layer.

I want to see Material B through the holes in Material A


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so I want to end up with a 3 layer material

hmmm... if you post some kind of a sample image it's a bit easier to understand what you are after and how to acheive it.

See how the yellow paint rim has worn away to reveal aluminum underneath?

What if the yellow material had an 80% yellow paint layer and a 20% additive shiny highlight layer and both needed to wear away to reveal metal?

Ok, I understand better now - Your first post is correct. It looks like you have three material definitions:
1) glossy surface
2) rough surface
3) aluminium

You need one weightmap for each type of surface.

I learned a lot about weightmaps by studying this example: http://www.maxwellrender.com/forum/viewtopic.php?t=21018

thank you