I want to make the scratches reveal shiny metal underneath, as if the scratch removed the paint. Is there some way I can use these maps to indicate metalness? Or can I use it to mix between a metal and paint texture? Thank you.
See alternatively my video https://youtu.be/-v-t_3oQzcs which shows how to create a texture that has metal showing thru paint work where scratched.
This is a slightly dated way of working but, it works - you can also use masking ID and PBR nodes and paint in Blender itself but, it is more complex and cannot be explained here. I will be releasing a video on it (or a series) sometime over next month.
Basic process (more detail in video) If you make a texture in photoshop or gimp (taking a UV export from Blender) make a base layer of a metal texture that you want the look of to show thru as the scratches. Make sure if any grain in metal is appropriate to scale of model. Then create a 'paint' layer over the top and using a brush eraser paint away the areas that you want to show thru as scratches. Save as png and add that to your texture image node in Blender.