You do not need to apply sunscreen to tattoos before using SOLIUS because UVB light does not penetrate the skin deep enough to affect any tattoos.
On average, a single treatment in SOLIUS exposes your skin to the same amount of UV energy as 9 seconds of sun exposure. Unlike the sun, the small amount of energy emitted by SOLIUS is in the UVB spectrum, which is primary absorbed by the epidermis (90%) leaving the tattoo ink in the dermis virtually untouched. A lifetime of SOLIUS use would have less impact on a tattoo than a single afternoon in the sun.
That said, if you choose to wear sunscreen, consider the fact that any area that you apply sunscreen to will not produce vitamin D, as sunscreen blocks UVB light, so the more sunscreen you apply, the less vitamin D you’ll likely make.