Bump map for only part of a material.

My little microwave is coming along slowly. However, I need some additional guidance with its textures.
My goal is to have a charred bumpy texture to affect only certain parts of the microwave, mainly the places with darkened burn marks.

Currently I am using a cloud texture for the bump map.
Also, the bump map and specular map do not appear to be working at all on the microwave itself and I don’t know why.

I have included a copy of a picture of the microwave and a copy of the blend file.

Thanks so much!

ReflectingDecision11.blend (1.28 MB)

I have actually had this same question, so I will input what I already know. Also, I will bookmark this page in case any other answer pops up that I like better!

First of all, you can only apply a bump map to an object by selecting the texture and placing it on the object. It is really annoying, because if you want it to bump in one part, and not in the other, you are basically screwed. So far the only two ways around them that I have found is to 1) Change the normal value up to make it rougher, and 2) If you are UV mapping it, which I assume you are, you can create an image file and “Draw” on it. Textures, colors, etc. Well, in that drawing that you place, you can layer it. (IE; leave the part you want flat black, and the ones you want bumped with the texture you want.) If you want me to, I can post a picture, or create a blend file for you, or something. But hopefully someone else comes up with a less annoying way to do it!

You use a stencil map.

Material has 3 textures in this order:
Base Texture
stencil map

The stencil map (disable color, enable stencil in the texture tab) is a 256 gray image, which decides the blending between the two textures.
Black is no blending, white is full blending.

Thanks for the help! The only thing Im not so keen on is that it causes distortions in darker areas if the microwave, and you can still see the bump on the lighter area of the microwave very lightly… Ill play with it more…