Honestly I never cover anything, Just make sure that everything is closed tight- Oil cap, Distributor and stuff like that.

I go to those do-it-yourself carwashes with a spray bottle of Simple Green and a toilet brush. Spray the simple green every where then brush and if you see a extra dirty spot repeat then start motor and rinse it clean. I have done it this way 20+ times and never once a problem and always clean.

Good luck