Sometimes it can be tough to get a clean-looking stencil with no visible bridges.
Stencils with more layers typically have fewer visible bridges
If you’re having trouble with awkward bridges, it might help to dial up the number of layers in your stencil.
The more colors (layers) you use, the more likely that you’re going to segment the shapes in the image into different blocks of color. Sure, some of those color blocks might still be intricate shapes, but in general the number of visible bridges goes down when the number of layers goes up.
In the two-layer stencil above (actually just black on a white background), the masking for the lightest part of the face has to be held in place by four bridges, whose outline you can see clearly (two on each side of the face). Without these bridges, that central bit of masking would not stay in place.
By comparison, in the 3-layer stencil, there is not the same distance to cover to bridge the white mask to the edge of the image, because you only have to bridge the white mask to the edge of the gray area, not the edge of the black area. The five- and eight-layer stencils are similar in that there are the same number of tones in the face; but in any case you see that–for the same image–the effect of the bridges becomes softer with the increase in the number of colors.
One more thing that affects the visibility of the bridges is that, in a two-layer stencil, the contrast between masked area and unmasked area is maximal: you see white where the bridge crosses the black area, and it’s super visible. On the other hand, the more colors you have, the less contrast there is likely to be between the paint color and the surface color. (The difference between gray and white is less than the difference between black and white.)
And by the way, it’s okay if you really, really like this stencil of Frida Kahlo with four quite visible bridges: it’s a stencil aesthetic that appeals to many. You can like it and still want to have some tricks up your sleeve for hiding bridges in other situations.
Reduce bridge concentration by giving the bridges more convenient points to attach
Suppose you’re having trouble with awkward bridges, and you really need to just use a single stencil to get the job done. One of the ways we can reduce the effect and visibility of bridges in single-layer stencils is to avoid including detail that requires remote masking in a wide expanse of the opposite color.
Take for example these two slider settings for our single-layer stencil of a BMW 3.0 CS (great classic car!). On the left, you see the dark underbody with the wheel and grill details emerging from darkness. You might be tempted to make this one just because it looks ‘heavy’ on the bottom. Fair. But on the right, with a slightly lighter interpretation, we’re going to end up with a much better of shot of connecting those masked-off details to the rest of the stencil without so many distracting bridges.
And here’s the comparison of the two images, in a simulated painting with a physical stencil that connects all the masks. Notice that in the dark, ‘heavy’ stencilization, we have to bridge all those wheel and grill details to the rest of the physical stencil to keep the whole thing in one piece. The version on the right isn’t perfect, but it actually looks like a car and not a bunch of cracked lines. The reason is that there is more light area for the bridges to ‘grab onto’ and the islands aren’t in such deep, wide seas that they take long bridges to connect them.