You’re doing well. There are a good number of ways of masking and mending a finish. With the Alclad, did you allow enough drying time? If the humidity was higher than normal you may have had a problem there. Depending on the line you described you may VERY LIGHTLY sand the affected area until you see the line disappear. Once You respray, using light coats, you should see a normal finish. Be careful not to sand too hard and bore a hole through the finish and through the primer to the bare plastic. For masking tape, I use the curved tape from Tamiya, to do the compound geometry of angular paint jobs. There are other curved tapes that are available and similar to Tamiya’s that do good work. Masking stencils are also available from Sprue Bros. online. Or, you can use existing masking tape, using a compass or template, and make your own curve, or shape. The “Blue Nosers” (352nd Fighter Group of the US Eighth Air Force) used a generic blue paint scheme (Sometimes the blue color appeared lighter, other P-51s were darker blue) once P-51Bs were first assigned to the group after transitioning from P-47 Thunderbolts in April of 1944. Study the photos of Lt. Col. Meyer’s “Petie 2nd” and you will see how to apply the masking tape on the nose section. I hope this will help you. Below is a photo of the masking I did on a 1932 vintage Curtiss P-6E biplane.