Buy an undercoating gun,pressure regulator,step drill bits,plugs(for the holes drilled) air compressor,rain suit and a mask.Add up that cost then figure out your better off paying to have it done.Be advised some shops will not service your vehicle after your done.I would but I also add 1-2 hours labor to any service if that has been done.
have done this, I use a hand pump garden sprayer,. Think about this,you oil the under,dirt road to dry it, now you have dirt stuck under your car/truck. when it rains(I know it has been a while hear in western pa)the dirt/ dust sticking to the under, is now wet/mud holding moisture next to your frame and under parts, I no longer do this because of what I have described .
why is it that when I see an older truck up here and I ask them how they keep it looking so good from rust the answer I always get is oil undercoating .someone pointed out to me there is a product called liquid film sold at the farm store and it is supposed to be very sticky ,when I find the needed sprayer I will give a full report
Bought a new car last fall. Paid the $1k to have it Zebarted with lifetime yearly touch ups since we keep our vehicles for at least 10 yrs. time will tell if its worth the $$ since it was the first time I used their product but there is a guarantee that all treated surfaces will not rust.