I recently did an IAF paint job for the A2A P-51 (posted on Simviation/FSX/Payware). I was probably having the same issues as you are. I wanted more control over the curves and the blurring of the edges that depict paint overspray. I used to do it manually, but that can make it difficult to get the overspray effect to match where different files meet.

What I do is create the camouflage pattern using vector graphics. I get the edges to line up perfectly before I blur the edges using trial-and-error between FSX and PSP/DXT. Once I get things positioned just right, I then "duplicate" the layer, turn off that original vector layer, then convert the new duplicate layer to raster.
After that I run through my SOP procedure of blurring the raster layer about 4 times using different settings. If I don't like what I see then I can delete the duplicate raster layer and start over again with the original vector layer group. This gives me total control over the situation.

I use Paintshop Pro, but should be the same techniquie with PS.

John