PBR - for grunge detail would you just increase the roughness or reduce the metalness as well?

e_manual

In the past I usual have just increased the roughness and changed the color to signify dirt / grunge when texturing,  since grunge is not inherently metal would it make sense to reduce the metalness as well? although I have noticed it oddly stands out against metal upon extreme angles, however could that be a more realistic effect?.

gnoop

In a game I am working for (custom engine)   it makes so ugly halo around metallic vs non metallic  pixels  I prefer to make just the whole thing either metal or not  or rather some flat shade of gray in metal channel .  After all you  can always compensate the issue in color channel.  

But it depends on texel density too.  More resolution you have lesser a problem it is .