I was spraying some Mr. Surfacer 500 on a tank turret to remove sanding scratches, but these cobweb-like things started appearing, gathering around areas with spaces, like the bustle rack. What happened?

sounds like its crinkleing as as build up of the surfacer i'd do it in thin coats at a time as the solvent in the gunze is reacting with the other layers as it's not getting a chance to dry completely. wait till it's dry sand it then apply it again