If they are real tiny pin holes they are likely solvent pop, which is caused by not enough time between coats for outgassing to occur. Subsequent coats trap the solvents in, where they will cause that damage once they finally break through. Urethanes are especially prone to this, the flash times must be followed religiously. Keep in mind, a "home booth" with inadequate air flow will adversely effect the outgassing where you'll need to add some time onto the mfr's recommended flash times.. Cooler temps, same thing.
