While painting a P-51B, I discovered a fairly easy way to mask those pesky wheel wells. What I do is airbrush the wheel wells and interior of the wheel well doors with appropriate colors. Let them cure for a day or so. Then use a small amount of Blue Tac or Fun Tak, inside the well interiors, and secure the doors over their respective openings. Once this is done, spray the appropriate color on the lower surfaces of the wings. When using 10-15 psi, I have never had a problem with overspray in the wheel wells or doors.
Mike M