Rattle Can Spray Paint Soft - How to Harden? Polishing?

[DoctorG]

Does anyone have any tips for Making Spray Paint Harden?
I find that its always quite soft, despite painting in light layers, and waiting an hour before recoat.
I have even tried baking small parts in the oven on low heat to avoid bubbling, but this doesnt seem to work.

Has anyone every used spray cans to paint something and found that certain colours like black are oily, soft, and easy to push a finger nail into?

Im finding that sanding and polishing is helping, but paint is still soft!

Thanks