The key to a good hard finish is to spray lots of fine layers and let the paint cure between layers. If you do a couple of thick layers it takes ages for the solvent to evaporate from the paint and thus harden, hence you being able to dig your nail in after a couple of days. Heat is good too, it makes the paint thinner and apply better, and also if the item you're spraying is warm it helps the paint to go off quicker.
remember lots of fine layers...
So its better to spray items in the summer really, unless I do them in the lounge! I think I will have a go at the radiator box idea, I didn't know that leaving the item for a few days/a week hardens it even more, cool. Like I said, im not bad at spraying and usually get a great finish but find I can easily dig my nail into the paint once it has been drying for a few days. Thanks for the info lads!