If it was me I would bake the spoiler off before applying any paints to it, as it could be the mould of the spoiler is not fully cured then add a barrier coat, something like bar coat then flat this off. Then on with the base and clear coat.
I know you didn't bake it last time and I wouldn't bake it after you have finished painting it just let it air dry.
Oh I am a painter of 17years experience, and have had this sort of thing happen to me before on an aftermarket spoiler as well.