If you sand off all the original paint, you can use any paint you like on the bass. You can also do oil too, that will look the most like the original finish if you liked its apperance and don't mind the care - I know some people that do enjoy that part, to each his own!
We no longer offer the satin natural finish that was originally supplied on Epics, the Orions, then Rogues then Excels. All basses and guitars now use our polyester finish, which is also plastic. This is the same finish that is on your Stanley Clarke bass, slapbass. Your Series I is either oil or a different polyurethane.
With the EPA and CA-EPA, it's not practical to shoot nitro in California as a business, so I don't have any experience with this. But I am curious slapbass - is it not possible clean the gun out before and after shooting a different coating?
Please take some before and after pictures of the project - I'm sure we'd all love to see the results of your efforts. If you send a picture to us with the serial number, we'll add it to the instrument's file.