Just to add something a little different in, I ran out of plastikote so I had a dig around in my garage and come across this ....
It smells the same so I give it ago, it seems all good. I'm not that sure how much cheaper it is than plastikote tho.
On the time left for backing I back mine pretty much straight after the paint job, I know people say you should let the paints dry out ect but I have been doing it like this for a while now with no ill affect, I just don't go to heavy on the first coat and flash off inbetween with a hair dryer, giving 2 or 3 coats in total ...