I have been told that it's not a great idea to paint body parts off the car. Panel's probably best placed to clarify but I believe a certain amount of blending needs to be done to get a professional finish. However, I have done it that way twice... I got a bonnet painted off the car for Trigger's Brush a good few years back (the man I got to do it was surprised to find I was leaving him the bonnet and not the car!) and even further back I painted a plastic bumper off the car myself for an Opel Astra, the one that was hit by a shopping trolley launched from Shantalla bridge. Can't say I ever noticed the lack of blending on either occasion but my motors aren't offices with filing cabinets, swivel chairs, water fountains and such like, they're just tools of the trade.
