Made for skin, not paint
Sunscreen is developed to go on and protect your skin, with all sorts of interesting additives to help shield you from the suns powerful UV rays. These ingredients however are not real paint or car friendly, which is annoying, as we will find them on all parts of the car after applying it in the car (especially if you have kids!). With most sunscreens being water resistant, normal washing will not remove them, and if they are left on for a long time (like many months), it can be virtually impossible to remove them by hand.
The reason for this is two of the main ingredients used in the formulas; namely polyethylene glycol and titanium dioxide. These work a treat at reflecting light and protecting us from UV, but when put on paint, it will leave that white residue and if it's left on the paint too long, it can damage the paint, requiring expensive machine buffing to remedy it.
The biggest tip we can give you is that the quicker you clean it off, the easier it will be to remove.