Lots of paint scrapers and special ones made to remove the collection of grease and dirt of a well maintained tractor of piece of equipment. A power washer is good to get some of it off too but there is a little risk of getting water where you don't want it. All kinds of soaps and solvents also help to remove it.
I had some extra stubborn stuff on my third restore a 1938 101 that I just could get it all with scrapers picks and the pressure washer. Asked a truck collector at work what he did it that case.
He said Oven cleaner nothing better and can be bought for a dollar at the dollar store.
I tried it and was very happy with the results. Becarefull though as it does remove paint too.