Hi,
How can I get a surface that allow portal shooting through it but also supporting painting (just like a hard light bridge would do)?

I tried with "%compilepassbullets" 1 in the material, but the paint blobs just pass through it (like it would do with a metal grate)

It depends on how you plan on implementing it, but it probably would just be best to use light bridges.

I know its not what you wanted but it would make more sense to the player if you just used bridges.

No, I don't want to use bridges... (they didn't exist in the 70's)
But that's not the point. I'm creating a series of tests, introducing a new testing element, some kind of solid field.
I now tried with all kind of invisible systems, but for some reason the paint sill won't work.

You'll notice that valve used a bridge in a 70's map. They might have stumbled into the same problem as you. If you are still detirmined i suggest you message Omnicoder.

Yes, Valve used bridges in old chambers but only in coop, because GLaDOS actually puts new testing elements in old chambers. Thanks anyway I'll probably ask to omnicoder.