Steps

Smudges

  • Add a new image texture node and load in the smudges texture (For this use the white smudges on a black background.)
  • Set the image texture node to non-color-data.
  • Create a new mapping node and feed that into the smudges texture node, this will allow you to make scaling changes later on.
  • Add in a Mix RGB node and connect it to the roughness map just prior to when it's inputted into the roughness input of the principled shader, be sure that the original roughness map is connected to the Color1 input of the Mix RGB node.
  • Connect the smudges texture to the Color2 input of the MixRGB node.
  • Set the mix type to screen and then adjust the mix amount of increase or decrease the strength of the effect.
  • Adjust scaling as needed via the mapping node.

Scatches

  • Add in a new image texture node and load in the scratches texture. (For this use the black scratches on a white blackground.)
  • Set the image texture node to non-color-data
  • Create a new mapping node and feed that into the scratches texture node, this will allow you to make scaling changes later on.
  • Add in a Bump node and feed in the existing normal map into the normal input of the bump map node. This will ensure the normal information from the original material is maintained.
  • Feed the scratches texture into the height input of the bump node. Adjust strength as required.
  • Adjust scaling as needed via the mapping node.

Links

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