Personally I was always told to skip undercoating. They never get 100 percent coverage and all it takes is one little spot without coverage and it is all for nothing. At that point all it does is hold in the water and road salt and it will be impossible for you to get it out and then you have a never ending corrosion problem. For me it is best to leave it as it came and wash the undercarriage periodically. I have owned many trucks and have never had any rust or corrosion problems. A buddy of mine had his new Chevy undercoated and it rusted out in under three years. Now, I'm sure others will have different opinions but this is mine and I'm sticking with it.
Probably areas that salt roads in the fall-winter-spring, where the salt gets applied to the under carriage, where I've seen car bodys rust out in about 5 years, this is where some sort of coating may be in order. But not many Class A's are being driven in those conditions since many head south in the winter. Remember Salt, Sand an dirt will wear off undercoat. You can get spray cans at Auto-Zone or Walley world to touch it up when needed.
1976 Newell Classic (Sold)
Home Base: Riverside, CA
If anyone needs my contact info private message me and I will send it to you.
Location: Just North of Detroit, a surprizingly great city
My 77 coach was developing surface rust spots on the framing and really rusting the rear wheel wells. Living in Michigan and leaving the area or arriving back in winter will expose those parts to salt. Washing is critical first line defense and I didn't do enough of that and hence my rust issues. Be absolutely sure that the separation film between the steel framing the aluminum skin is present or you will be amazed at how fast pinholes appear in the aluminum following exposure to salt.
I have replaced the deteriorated poly separating sheet with heavier shrink wrap film, wire brushed rusty spots, treated those spots with rust converter and topped it off with an oily undercoating. I don't have a lot of miles on it since so I can't verify durability. In my earlier life I would just spray a mixture of used engine oil and diesel fuel on rusty spots and that seemed to work quite well. It was cheap, penetrated into nooks and crannies, and was messy to install.
I've also put a rubber roof flashing on some more effected spots in the rear wheel wells. It is self-adhesive but it didn't stick very well so I used rubberized roofing tar as glue - so far so good as it is still in place.
Jon and Alie Kabbe
Started with 77 Coach
Now have 39' 93 coach
2007 civic toad