Oh, turns out you cannot use Spec workflow in UE4 (even if you put spec to 1, that spec has builtin fresnel) Oh well, now that I use the keywords in your post "edge artifacts PBR" I can see that many threads discuss this issue already :)
Levus: Yeah maybe I'll put more dirt, we'll see. Spitfire: Thanks man. Like what ged pointed out, the spec needs more work. Ged: Ok I'll try it out. But isn't spec maps are supposed to be grayscale? Is it fine to use colored spec maps? Thanks.
Almost there! Nice beast :) You could try and break up the spec on the green sections flat parts, what you seem to have now basically a flat spec, add some speckle/spackle/contrast on the spec itself so that it matches up with the mild bumpages and usage around it. Good job!