Consider a mix shader node in your shader nodes where you can change the contribution of each texture. A texture can determine the contribution pixel by pixel. Two standard standard textures linked to Two BSDF shaders linked to a mixed shader. The relative contributions are mixed in a ratio specified by a hand painted texture. Image above.
Texture slot ...
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.
This should be the option you want.
Angle follows the direction of the brush stroke. Not available with 3D textures. (Shortcut sculpting only.)
If you've got tablet pressure enabled for the brush strength, but you're using a mouse instead of a tablet, then nothing happens because the mouse has zero pressure, and so the brush has 0 strength. I was confused by this as flood fill was working fine, but the brush didn't seem to be doing anything.
Alright , so this is as close as I got for today... I just saw you gave an in depth answer so first I have to thank you for that and I will try the tomorrow or the next day and let you know how that works out.
So for this image I figured out a way to mix stencils to paint this plane , which is almost exactly what, the only thing I need to figure out is how ...
The answer is that no, you cannot set a threshold that will limit the full opacity of overlapping strokes - these strokes from your example are additive, so the overlapping area appears more opaque than the parts that do not intersect.
The better way to handle this is to use the Mask tool to paint your strokes using the visualization color of your choice ...