Mike if I was you i would defo go out and invest in some Panel Wipe.....available from any Good MOTOR FACTORS yes I know it will cost you around a Tenner BUT it is well worth it,wash down your Plasticard before use,this is to get rid of any residue on the plastic,yes I do fully realise that you use Meths BUT if Meths was used for degreasing well everyone in the Motor trade would use it for degreasong cars prior to spraying.Also tbh with you washing it with detergent is also poo poo as previous sentence states !!!!!!!
Now what has caused your paint reaction well it is down to a couple of causes really.It could well be residue left over from cleaning,it could well be that as it was (MAYBEE ) the first part of the degreasing that was sprayed that on the first pass of the aerosol you got a concentration of thinners,Had that today when using black as a guide coat on a car bonnet even though it was shaken for 2 Min's.
As for not sanding the plastic in anyway well sorry to say this and I know the well it works for me brigade will have a pop You SHOULD allways prepare your artice for paining in the appropriate way.It should have been sanded with either 600 Wet Dry or even a Grey Scotchbrite abrasive pad...reason I say this is simple really.If it is not prepped properly the paint will eventually come off your model in sheets...........OK you might think well thats how I do it anyway ............I know I am right in what I say .
My Thoughts on what has happened on your superstructure is bad Prep and Solvent attack sorry to be so blunt but hey ho such is life
Dave