Simple answer is it depends!

First of all you need to treat whats causing the bubble as if you don't and you just paint over it, it will just blister up again. If it is a steel winged car, I'd be thinking there is a bit of corrosion going under the paint!

Another factor is what the car was painted in and how long ago? Its rare these days but not unheard of to get a paint reaction due to two dissimilar paints!

Also bear in mind heat and humidity, I'd advise against an outdoor paint job at present as it will probably bloom up due to cold or if its humid, you run the risk of micro blistering due to damp being trapped between coats of paint. The funny thing about microblistering is it can start to show years after the paint was put on. If you can paint in a well heated garage all the better.

I've nothing against aerosols for a basic job. Quick and easy and mess free - compared with stripping and cleaning a spraygun after use (and before two pack paint goes hard in it!) Keep the nozzle parallel with the surface to be sprayed and all will be well, start "arcing" the nozzle and you end up with too much paint at the ends of the job and not enough in the middle and too much paint equals runs!!

Surface prep is essential - panel wipe is a must to remove wax and silicone as are tack cloths between every bout of sanding!!

A really good book is Lindsay Porters "Car Body Work Repair Manual" - think its out of print but readily available though!

Sorry its an essay but hope it helps!!!

Arwyn