Re: What caused this?
the quality of a topcoat can not exceed it's ability to adhere to the substrate upon which it's applied.
I've never seen where alcohol was an effective solvent for cleaning anything related to mechanical parts.
use lacquer thinner or acetone.
...good idea to wet sand w 3k then shoot a clear over the top... if the paint is stable.
far better quality product available when u get away from the rattle cans... with a very few exceptions.
even the cheapest airbrushes available are superior to rattle cans... and u can run one off a car's spare tire !
as said above, it's possible that your last spray pass, after everything had good coverage, and a nice wetted surface, was held too far and was too lite, possibly causing the atomized spray to dryfall on the surface leaving a bit of coarseness to the topcoat.
...I'm frequently guilty of doing that... just one more little bit... and it botches a job already well done! lol
paintwork is great fun! a difficult skill to master, but results are almost always an improvement. :-)