get the engine nice and warm, spray chosen product all over, allow to soak for 5/10 mins reaply in between so it doesnt dry out, start engine and spray off detergent, be carefull of your earth cable on the engine, alarm sounder box,coilpack,dizzy cap, alternator will be fine if youve got the engine running and you dont drown it.
close bonnet,leave engine running for 15/30 mins and it should be dry enough for you to spray some dressing on.