Best tool to clean/remove years of oil/dirt buildup?

I did all of the above, but the best tool I found was using a needle scaler. You will need a decent air compressor to run the scaler, mine is a 60 gallon upright. I didn't treat the undercoating at all, just hit it dry with the scaler and it pulled all that crap off. It is hugely messy however and tiring as you're using a mini-jackhammer overhead that is heavy and pushing back at you while you hold it in place. I had to take my time but it really was effective. After that I did the easy-off bath to get the lesser crud or in places where I couldn't get the needle scaler. As stated above, this process is no fun.