Yea defidently sand it (Use an electric sander, its hard to sand a guitar, your average guitar has usually two to four coats....)
I would take off the neck, as for your wiring problem, carefully cover all areas not to be painted with tape, so you don't need to worry about getting paint on them and then you can just take off the tape and no paint will be on the pickups ect
(use electric tape and make sure you get all the nooks and crannies on you bass!)
Will it be ready tommorrow?
Depends on the paint, but I would use the better quality, even if it takes longer to dry [We're talking about a bass here the coolest instrument in the world
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EDIT:Take off the knobs and cover the pot shaft!
And if your pickups are connected to the pickguard, take off the pickguard and move it aside, it takes less time then to cover with tape. Make sure to put a prtective coating after it dries too!
Make sure to use even strokes, left to right, and right to left, there is an art to spraypainting just ask one of those "homies" out there!
Obviously take off the strings....
Good luck me friend!