Originally Posted by jenn

Yes. You'd need a very thick even layer to get the labelled protection, which is not possible, and even then the UVA protection would be very poor (and it's the UVA rays that are responsible for skin aging and sun damage). You find info like that if you register on makeupalley and read the notepad by member sunscreenFAQ. Lots of great info based on scientific research can be found there- also on retinoids and antioxidants (there's a notepad by a member called Cho_Chang that links to all information). Lots to learn, very interesting- I spent the last years researching for fun and can totally relate to those girls.
It's a lot to read in the beginning- but it all is pretty simple in the end.
A helpful book is the "Skintype Solution" by Leslie Baumann.