Applying a custom paint job to an upright bass can be a great way to stand out from the crowd and also impart some personality to your instrument. While some cautionary steps need to be taken to protect the integrity of the instrument, there's little holding you back from giving a fresh coat to a bass and injecting some new life into it.
Remove all pieces of hardware, strings and anything else that isn't going to be painted on the instrument.
Cover the fingerboard of the bass with painter's tape to protect it from paint. Also use some tape to fill in the f-holes to prevent paint from getting inside the bass.
Sand lightly the entirety of the instrument, making sure that it is as smooth as possible all over.
Apply multiple coats of primer to the instrument. If desired, let each coat dry and then lightly sand the instrument again to ensure maximum smoothness.
Using a brush, paint the bass, taking care to make sure the coat is applied evenly and smoothly. Like with the primer, you can let each coat dry and sand it lightly before applying a new coat.
Apply several coats of lacquer to the dried painted bass to protect it.
Be sure you have a clear plan for how you want the finished product to look before you begin painting. For a smoother, more even paint job, you could use a paint sprayer instead of using a brush.
Be sure to work in a well-ventilated area while painting, and wear a protective mask if using a sprayer.