In the "reduce, reuse, recycle" department, here's something I learned about degreasing engine compartments, particularly on the kind of Hopelessly Shot Old British Sports Cars with which I've so often messed about.
If you usea trigger-type sprayer to apply degreaser, at least on a really grimy engine bay, you'll end up with carpal tunnel syndrome in like nine and a half minutes. So I came up with a neat hack:
1. Take an empty 2L soda bottle. (If it had something sticky in it, clean it out, or use an empty club soda bottle.) Reserve the plastic bottle cap.
2. With a finishing nail or a small awl, punch a small hole in the cap, about 1mm across.
3. Fill the 2L bottle with a mix of hot water and Simple Green cleaning concentrate. Adjust the proportion of concentrate to water depending on grime level; I used a ratio of about 3 parts water/1 part concentrate for the engine bay of an MGB.
4. Hold the soda bottle in one hand and apply light pressure to the middle of the bottle. This results in a constant stream of hot water-degreasing compound which you can direct right onto the area you're working on. To stop the stream, release your pressure on the bottle.
5. When the grease/cooked-on oil has started to soften, use a plastic Bondo spreader to scrape it off the sides of the engine bay, the crossmember, etc. (If you start off using a sponge, it will just get impregnated with grease and oil in the first pass.) Keep up a steady but light stream as you scrape, to soften each successive layer while you work.
6. Once you have removed the baked-on goo, THEN you can use shop rags or sponges to wipe the paint clean.
Simple Green works very well for this, as it's available in concentrate form, is biodegradable and non-toxic, and leaves your car's engine bay with the fresh scent of sassafras, the herb used to make filé powder for adding to your gumbo. Laissez les bons temps rouler!