I haven't had a trawler engine to maintain yet but one of my maintenance theories is that if you can prevent something from deteriorating in the first place you are better off than trying to refinish it. To that end, I clean my sailboat engine with every oil change. Soap and water are my tools here. After everything is clean, I run the engine and get it warm. Finally, I empty a can of WD-40 on the engine and mop everything up with a clean rag. This 11 year old Yanmar has about 2000 hours on it and it looks like new! I do all of my own maintenance and it gets a valve adjustment annually, a water pump impeller, a complete check of the wiring harness including connectors, antifreeze replacement, fuel filters etc, etc. She runs like new!
The tin foil trick is a good idea though and I will remember it for the future!