Using aluminium foil to mask items you don't want sprayed is a very simple and cost effective way to mask under hood for firewall painting or engine compartment painting.

Here's a helpful tip for masking under hood components when spray painting your engine compartment.

Use aluminium foil to bend around and wrap anything you don't want over spray on.  Its fast and stays put until you remove it.

This tip came from many years inside of body shops.  Add this to your bag of tricks and you will appreciate it.