If you want a nice job you need to add adhesion promoter and a flexy additive - otherwise the paint will tend to crack and fall off
Beat me too it, painting door trims with standard spray is ok as they don't flex much but a bumper would require a flexy additive to the spray so it can, like it says, flex. If you use a normal spray can off the shelves on a bumper and it takes a knock it'll just crack off and look terrible!
I know Halfords can mix colours up and 'make' a spray can for colours not on the shelves but I don't know if they can add the flexible additive to it so it can be used for bumpers etc, might be worth a call to them ??
Also don't mix a synthetic and celloluse paints together as you'll get a very not attractive finish as they will react with one another and you'll get bad bubbling and peeling and it'll all have to come off and started over again.
The hair driver effect is good but be careful not to heat the paint your drying too much as again it'll bubble.
Lastly - Health & Safety.
If spraying in an confidend area make sure you use eye protection and more importantly a good air mask. Don't use one of them flimsly paper ones as they are rubbish and they won't keep paint fumes out. Concentrated paint fumes can kill so never
be tight on the level of resportatory protection you buy, you live could depend on it