Yes bumper ..... (XD on the Camel comment, the idea is to show case the solution not any form of toes ...)Well, since you're not a camel, then your toe is not that popular.
I would try to have the first coat of primer on it within 10 minutes after sanding. I would also try to sand and paint in small sections rather than the entire piece all at once, at least for the etching primer. Once the primer is on, then you can recoat or top coat within the time frame listed on the can.
Also, for clarification, I believe you are referring to an aluminum bumper, not a fender.
As @Dusten said, for aircraft, they work within 12 hours of sanding that it must be reworked. Aircraft has a higher grade of aluminum alloy and will oxidize slower than the softer aluminum that you most likely have on your bumper. So, the sooner, the better.
Yah in that case it can be divided into pieces sand and paint then move on the next section.
I want to learn the process so will eventually do it myself in all cases its not going to be worse then what I have (I hope!)
Cheers