The contaminated rag scenario sounds the most likely.
40 psi sounds o.k, although you can take it to 35. ( I prefer 40 to 50 myself)
You mentioned that you noticed the silicon fish-eyes popping up on the fourth coat.
This can be compounded if there are quite a few coats applied in quicker succession than
would be recommended. Remembering that you want your previous coats to "flash off'
before you re-apply over the top. In essence, give a bit more time between coats for drying.
As far as a degreaser goes, I would consult the can or what have you, for a recommended product
for that particular brand. The products website could have data sheets in this area.
Always degrease before spraying. Even just to remove body oils from handling the item.
We used Prepsol which was perfect for Automotive applications but maybe not for yourself.
One more thing !!
If you notice these fish eyes appearing while spraying again, let it dry out for longer before
reapplying a dust coat over the affected area.
Again this is there to seal off and hopefully stave off further reactions from permeating future coats.
Then you should be able to reapply as a normal coat.
The normal reaction is to spray on heavy coats to try to bury it. This just makes it worse.
So , take your time and go lightly !!
Good luck with it and let me know how things go.