Spit-shine Collinite over Nattys is my personal fav. You get the deep wetness (ahem....) of the nattys blue and the durability and clarity of the Collinite. Last spring/summer i did the same but with Harlys instead of Collinite and that is also a good combo.
As for how long you wana leave it on for?? Personal preference is on/off specially during the summer with the sun and all. If you do however feel the there is some benifit to leaving it on for a spell, then use a mist of your favorite QD on the MF, tends to stop any dusting you can sometimes get