It's not obligatory, but power washing it will remove dust, dirt, some mold.

I would just try to use as little water as possible, do it when the sun is out so it can dry faster, and then sand it.

And after sanding it I would clean it using paint thinner or at least alcohol.

The song is intended to sound cynical and a bit annoying; the way Harper sings is part of the critique to the musical industry.