When it comes to wheel arches, I use Omikron RD Black. A spirit based satin finish product that you spray on. I put some on every week I detail and eventually you have enough on there that the arches get a really wet look finish. Looks awesome. The only drawback is that if you got it on paint, virtually nothing will get it off. Don't use on a windy day. I also use this for the inside of my car's chrome exhaust tip to prevent that horrible orange colored rust that seems to strike most cars. Great for exhaust mounting rubbers and the engine bay.
For underbody it's Lanotec Lanolin. Comes in aerosol or 5 litre container.