Sunless solution does not contain any Sun Protection Factor (SPF). It therefore can never be considered sufficient protection against UVA/B radiation (sunrays).
Remember: it is always important to protect your skin during any sort of exposure to UVA/B rays; sunscreen is always a must! Applying a lotion-type sunscreen that is non-greasy will not only protect your skin when basking in the sun, but also provide critical hydration to the epidermal layer of your skin.
If you are going to be in the water, or prefer applying your SPF with an aerosol application, try a breathable sunscreen that won’t clog pores and is fast absorbing. Be sure your client knows that their spray tan will not provide any sun protection, and to avoid rubbing the skin when rinsing at the end of the day. Remember to try to stay out of pools or highly chlorinated waters as it will slowly remove your new amazing spray tan.
Happy (safe) tanning!