Paint Cracking Problem

I am in the process of building up a F-86F Sabre and plan on putting down various shades of Alclad II metallic paints on it. I first airbrushed a coat of Model Master 36270 gray as a primer coat, using Testors airbrush thinner. I let that dry for 24 hrs, and then applied a coat of Model Master Gloss Black. I let that dry for 24 hrs and yesterday got ready to apply a second coat when I noticed a couple places where the paint has cracked, small cracks and wrinkles to be sure but would definitely be noticeable should I apply Alclad without sanding. I haven't run into this problem before, but am guessing that I thinned the Gloss Black too much. Anyone have any ideas? I plan on letting the Gloss Black thoroughly dry, probably about a week, and then sand out the cracks and wrinkles with 2400 to 12000 sanding pads before attempting to put down another coat of Gloss Black.

Thanks in advance,

Dick McC