Every ruined stencil job dies the same death: the can was in the wrong place. Not the wrong paint, not the wrong stencil, not the wrong wall. The hand holding the can drifted eight inches closer or a foot further away, and the crisp edge you cut turned into fog. Distance is the whole game, and the good news is the rule fits on a sticky note.
The working range: 6 to 10 inches
Hold the can 6 to 10 inches from the stencil. Closer than 6 and the propellant blasts wet paint under your edges and floods the openings. Further than 10 and the paint half-dries on the way in, landing as gritty dust that reads gray instead of black.
Inside that range, pick your spot by the work. Fine detail and halftone dots want the near end, 6 to 7 inches, with quick passes. Big fills can live at 9 or 10. If you only remember one number, make it 8.
Keep the can square to the wall
The can stays perpendicular to the surface. Straight on, nozzle pointing flat at the wall, every pass. The moment you angle the can, paint approaches the stencil edge sideways and slides underneath it. That is where blowouts come from, not from bad stencils.
Move your whole arm, not your wrist. A wrist flick swings the can through an arc, which changes both the distance and the angle at the ends of every stroke. Lock the wrist, sweep with the arm, and the can holds one distance for the whole stroke.
Start off the stencil, finish off the stencil
Cans spit when the valve opens. Begin each pass in dead air past the edge of your stencil, sweep across, and release past the other edge. The sputter lands on nothing, and the stencil only ever sees the smooth middle of the stroke.
Light passes beat one heavy coat
One slow, soaking pass will bleed. Three fast, light passes will not. Each pass should look too thin on its own. Give the paint 30 to 60 seconds to flash between passes, then hit it again. You are building opacity in layers, and the layers dry before they can creep under the mylar.
Test on cardboard first
Every can sprays a little different, and the same can sprays different on a cold morning than a hot afternoon. Before you touch the real surface, fire two passes at a scrap of cardboard from your working distance. Ten seconds of testing tells you the fan width, the wetness, and whether the nozzle is clogged, while it still costs nothing.
The recap
Eight inches out, can square to the wall, start and finish off the stencil, several light passes with flash time between, test card first. Do those five things and the edge that comes off the wall is the edge you cut.
Grab a halftone stencil and a can from the spray paint shelf and put the rule to work.
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