i've been using giorgio armani luminous silk foundation and pressed powder, but today i've decided to try out laura mercier's mineral powder makeup, bronzer, and blending brush. i also bought the mineral primer and brush...for those of you who use LM's mineral makeup, do you use mineral primer beforehand? is this a necessary step or can i forgo using it? TIA! btw, what's the story behind bismuth oxychloride in mineral makeup?