I’ve done bodywork in Vermont for 40 years. Many rusty vehicles. Basically, if u don’t get the protection in the areas needed, nothing works. Oil undercoating is popular in our area on vehicles four or five years old, or older. New vehicles are usually done by the dealers. Usually those situations don’t get into areas needed, like rocker panels, quarter panels, etc. on my vehicles, I use a wax based product by 3M or Sem, and use a wand to get into tight areas. If u get it where it’s needed, it works, but u need to do it periodically. Also, u can’t substitute for keeping a car as clean as possible. Sometimes hard in Winter, but it is important. Every Spring I crawl under my truck and hose out the wheel arches, rocker panels, cab corners, etc, to get all the sand and salt out of those area from Winter. You vehicle will last longer if u do this! I see trucks that need rocker panels, and box sides in five years if not kept clean and undercoated “well”. You can have undercoated, but if not done well, it’s wasted money. I see it every day. A squirt here, a squirt there. That’s not going to help anyone, but unfortunately you may never know until it’s to late. If u can’t do it yourself, go to someone you trust! There is a lot on the line. Vehicles aren’t cheap.