How to mask normal maps or How to select and apply portions of normal map? I have normal map that is fed to "Diffuse and Glossy" shaders. but I don't want to use or mask portion of the normal map. 

How to mask portion of the normal map or in other words how to apply portion of the normal map ? 

I have tried combining texture painted image and normal map with MixRGB: Multiply to select  desired portions of normal map by painting white. but, it's no use. As shown below
In comparison to bump maps were black and white are 0% and 100% influence, normal maps work from a blueish base tone.

rgb (128,128,256): is 0% normal map influence


you can use your bw-mask as a factorial in the color-mix-node while using your normal map as one input and the rgb color(128,128,256) as a second input.