When you open the default FSX texture for "Lear45_T.dds" in DXT Bmp you can "Export the alpha"...And save it.
Then when you get the painting done in PhotoShop You can add the 'Alpha' in PhotoShop...

Then once you add the 'Alpha Channel' open the Alpha you saved from DXT Bmp in PhotoShop and ctrl-c the 'Alpha' then ctrl-v into the 'Alpha Channel' you made in your repaint then save....
So you now you see why I hate defaults....almost everything is mirrored.
One fuse side one,wing..Etc...
Jeff