for small detail areas I like to use liquid masking tape and or frisket. Find at art supply stores.
If you use the liquid mask paint it on and when dry cut with a film cutter, or even a xacto blade with a light hand peel off from the area you want to paint, them use the area still masked to apply masking tape and paper to cover other areas.
In a bind use contact paper. As above it cood to pull the tape before paint is cured out, also cut the paint film with a sharp exacto/light hand to avoid lifting.