Exfoliating beforehand is one of the more reliable habits for a smoother, more even, longer-lasting tan, regardless of whether the color comes from sun exposure or a self-tanning product.
Why dead skin interferes with even color
The outermost skin layer sheds unevenly across the body, and both natural tanning and self-tanner interact with that top layer — starting with a smoother, more uniform surface produces more uniform results.
Timing matters
Exfoliating the day before (or same-day, well before exposure) rather than immediately beforehand avoids irritating freshly sensitized skin right before UV exposure — a gap of several hours to a day is generally reasonable.
Don't over-exfoliate
Aggressive daily exfoliation can compromise the skin barrier and increase sun sensitivity — a moderate, gentle approach a day or two before sun exposure is more effective than doing it right up to (or during) a tanning session.
Is it fine to exfoliate the same day as sun exposure?
Generally yes if done well before (morning exfoliation for afternoon sun, for example) with a gentle method — avoid harsh exfoliation immediately before stepping into strong sun, since freshly exfoliated skin can be more sun-sensitive.