I haven't had this problem with Tamiya Gun metal, but clearly something went wrong. Things like this have only happened to me when I've put down too much on the color or clear, but you said you tried to keep things light so I don't know what to tell you.
I don't think you can fix this with sanding. Looks like you might have to strip the thing.
One thing I would do in your situation is test the paint on a plastic spoon, then go over it with clear and see if the same thing happens again.