You mean you’re using an inverted metal map as specular map?
I really don’t understand how specular works in UE4. I don’t even understand why this input is here in a roughness/metalness workflow engine and if it has to be kept at default 0.5 value.
Leaving it at 0.5 makes 1.0 rough materials (that doesn’t exist in real world, not even for Vantablack) to reflect a lot of light. Everything looks washed out/dusty.
A landscape grass material set to 1.0 rough is unrealisticly reflective : https://80.lv/articles/alireza-khaje…-and-lighting/
Also, reducing the roughness of the material makes it look less washed out/dusty.