I've never liked using armor all or anything on my engine bay due to slipping belts and maybe not so safe on electrical connectors...
WD-40 is the shit, dust off as much as you can that way it does not clump up... Spray the entire bay (metal and all) with WD, let it sit about 5 min then wipe it down. SHIT WILL LOOK AMAZING!!!!
Just though I would share, if it's a re-post sorry...
Here are some pics... A before, an after, and a now just because lol