What I use for undercoating and for touch up is stuff you get by the gallon at farm stores called "plow coat" looks like tar but is liquid like water, I use a pressure pot sprayer setup to spray it on, I spray a few light coats of the stuff it will melt into the old stuff the solvents will bond to it, messy stuff but works well, I let that dry a few days then go over it again with "foundation sealer" you get it in 5 gallon buckets at like home depot, its thick so I thin it with the plow coat until I can spray it on, you want it keep it thick as you can so you can build up thickness, again the whole thing is a total mess and it smells, what ever you spray it on will smell but only for a week or two depending on the weather, if its hot and dry it only takes a few days for the solvents to evaporate.
I bought the cheapo do it yourself undercoating kit from JC witney it works great I treat inside the doors and rockers too, the undercoat stuff they sell last about one year then it falls off.
This mixture is by far the best I have seen yet the stuff bonds and makes a water tight seal I have seen it last 10 years in wheel wells in hard Illinois winters.
When I apply this I wear cloths I am going to throw away when I am done no matter how hard you try to stay clean you get it on you :- ) Last time I did this I did my truck suburban and motor home all at one time, the bus I just bought needs a good coating so this summer I will give it a good coating.
Good luck.
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