Mark, to get a clean line you need a quality masking tape. Great Planes, Duratrax, Tamiya, all offer vinyl masking tapes in various widths. Auto painting supplies have them.
For tight curves (like the red scallops on the wheelpants) you need something no wider than 1/8"
These tapes go around curves really well and stay stuck. Of course, the surface prep is very important... surface must be absolutely clean and smooth and the paint you are masking off must be fully set.
After drawing out the area to masked (lightly with a lead pencil) I then mask it using one of these narrow masking tapes. It also helps if this is being done in warmer temperatures as everything is more flexible. I use the narrow tape to mask off the area to be painted, I then use a combination of blue painters and regular masking tape to protect the areas from the edge of the narrow flexible vinyl tape, outward.
You also need a large roll of painters masking paper (ACE hardware, etc). It is very thin brown paper and it tears easily and cleanly. It is also very dense, and will not allow paint to get through.
Use this paper to protect everything else, using regular tan masking tape to secure. You REALLY have to be generous with the paper and tape to protect EVERTHING... failure to do so will reveal areas where paint found it's way past and onto your project where you don't want it!
I am using Enamels (ACE, Krylon, Dupli-Color, Rustoleum) that dry to the touch in about 15-30 minutes. You want to remove the mask as soon as the paint is dry to the touch, but NOT fully set up. The tape should be pulled back against itself and slightly away from the newly painted surface...
Perhaps when I paint the scallops on the wings, I will take more pictures of this process so that my explanation is perfectly clear...
The most difficult part about using "rattle cans" for painting is keeping a 'wet edge' especially when spraying large areas... it can be almost impossible to meet up where you started and still maintain a wet edge.
To correct these areas that show over spray (a dull powdery appearance because the paint had already begun to set and the newly applied paint "dusts" the glossy areas), I often have to go back with auto polish and vigorusly remove the dusting of paint to reveal a gloss. It doesn't always work perfectly... depends on the paint, the polish, etc.
I have also had to deal with the "FUN" of having my surface primed and perfectly prepped, only to have a brand new can of paint puking out small 'paint balls' and splatters!! When that happens, the only word that comes to mind is MURDER!! But, what I have had to do is wait for the crappy paint to set up enough to sand it (15 minutes) then hit it with 400-600 grit and water. It can be messy and make a person wonder *** they are putting themselves through this??!
Oh, and I forget to mention that a TACK RAG is also absolutely essential as is DIRECT SUNLIGHT in making sure the surface is free of debris AND the paint being applied is covering 100%, but not overly applied. This means little to no wind. Yes.... it's almost as if the stars must all align for a quality outcome
As I do not have a paint booth, painting airplanes is ALWAYS a very challenging endeavor!
In the end, it is worth the effort IF your project turns out nice.