I did mine today...sprayed everything with Meguires de-greaser, brushed it in to lift the dirt, then foamed the whole engine bay with my foam lance and washed it off with my power washer.
I didn't cover anything with cling film...it's an engine...it sits gettig sprayed with water all year round...anything that needs to be water tight, should be water tight!
I've done the same for the last 23 years...and never had a problem.
The old ABF started first turn of the key afterwards too.