The first rule of applying sunscreen - The spf's of various products don't add up!!! If you are applying a moisturiser with spf, say x, then a sunscreen / foundation / BB cream with spf, say y, and then finish off with a mineral compact which again provides some spf, say z, factor, you can't say that the sum total of your sun protection factor is spf x+y+z.
But, something else bothers me in this rule!!! Well, when we have layers of product with spf factors, x, y or z, which spf does apply on the skin - x, y or z? Or, does it work like a filtering mechanism? Meaning, if supposing the top layer has spf of z, those many rays are allowed to pass (remember even 50 spf allows 2% sun rays to pass through, it provides around 98% protection!!). And, then, the next product spf, which is y, again filters out the rays according to its strength and so on!!
Does that happen at all? Well. logically speaking, it does seem quite probable but I don't know how much of that really applies in practice. Would be great if some sunscreen expert can answer such questions!!! What are your thoughts on it?