It's very common for a stencil to bleed if these few steps aren't followed :
- Stick the stencil firmly
- Heat the stencil (be careful not to overheat if you stick the stencil on paint)
- Press the stencil with a card
- Apply a base coat of paint or primer
- Never overload your brush or sponge
- Apply thin coats
- Respect the drying times