Does treatment change SPF enough that I have to re-test?

Yes, and the direction depends on the agent and the loading. It cannot be assumed either way — it has to be measured.

Two mechanisms act at once. Better dispersion means the particles sit more evenly through the film, which usually raises UV shielding efficiency. The treatment layer itself can absorb in the UV range or alter refraction at the particle surface, which can nudge efficiency down or up.

The same particle size in different treatments can therefore give different SPF. Our grades include N15E (alumina plus stearic acid), N23M (silica plus triethoxycaprylylsilane), Z-45 AS (triethoxycaprylylsilane) and Z-45 DSG (disodium stearoyl glutamate) at comparable sizes.

When you change treatment spec, re-run in-vitro SPF on the same formula and do not carry the old data over. Test UVA as well — PA or critical wavelength — because treatment need not affect UVA and UVB the same way. If the product is already notified, a change of treatment is a change of raw material and may require re-assessment.