I've always wet the outside of the hood, fenders,grille... Then sprayed Simple Green around the engine bay and the underside of the hood. Let it sit but not long enough to dry and a garden hose will work ok. I've also used a power washer but avoid spraying anything electrical with up close direct pressure, I learned that the hard way a long time ago. Simple Green is non-toxic, so they say, but I've never had it hurt my paint. Worst it ever did was remove some wax in spots.