The beauty of a spray tan is that it allows you to get full-body colour without any tan lines — but that's only if you get completely undressed.
"Whatever you leave on, you will have a tan line, so I always recommend going naked," said Sophie Evans, St. Tropez skin finishing expert. But you have options. "Most people wear the tiny disposable underwear provided," Evans said. "This allows the spray tanner to lift up and get rid of any lines if need be. If you leave your own underwear on, this can't happen and will leave unwanted tan lines."
If you do decide to leave on your underwear for the session, make sure it provides as little coverage as possible and you don't care if it gets stained.