I've never worried about putting it on in the sunshine.
Ours stay in the shade during the hottest part of the day. I've never had any problems with it sunburning. Only used it on black/dark horses though.
My filly Shiloh has had an allergic reaction to something out in the pasture. She had nasty welts all down her legs, chest and neck. I have been giving her feed on antihisamine and dousing her with MTG its really helping. Stopped her from itching herself so badly.
Gotta love that MTG