M.L.
i don't know what kind of mineral foundation you use, but i use Avon's mineral powder foundation (loose) and also their "primer" that is SPF 15. I went to the beach last week and my shoulders and back fried but my face was perfect. It's a great foundation & primer and they're pretty cheap. I don't sell it, I just use it :)
My friend also started using a different mineral powder foundation that she says is spectacular (she's in the spa industry so she knows her products). It is like SPF 50 and is also waterproof! i didn't believe it until i saw it and it really is! but it's like $50 a jar. but if you buy foundation AND sunscreen, maybe that amount wouldn't seem that bad if you only have to apply one product! Message me if you want the info on it (I can't remember the name so I would have to ask her).