Mixing suncream illogically

[magratvonlipwig]

Help please! Can anyone confirm or debunk my logic.
Every year i have a load of half used suncreams of varying factors. To save space i combine some if them into one bottle. I allways tip higher factor into lower bottle. So say tip some factor 50 into a third full bottle of factor 30. So anyone using the bottle gets at least the protection it says, if not more. In my head, this mixed bottle has a factor of about 40 ie ive taken an average.
But then, if i have put random face cream on, with no spf, then add some factor 30 afterwards... I assume i have factor 30 protection on my face. By my previous logic its diluted and only worth factor 15.
What do you think?

This is wrong.
A suncream has a very high number of chemicals and some are not compatible with others so mixing several ones in one bottle can be not only dangerous but they might also invalidate each other.

Thank you .... that never even occurred to me !!!

...probably because it isn't going to happen - mixed suncreams do not invalidate each other! Yes, suncream contains lots of chemicals. Life contains lots of chemicals. YOU contain lots of chemicals.

work on the lowest protection. There isn't actually a lot of difference between factor 30 and factor 50 if you do the maths, the extra drops away exponentially.

as always; no deliberate sunbathing, in the shade 11-3, wear clothes, enjoy holiday.