Kerosene is what they use as "thinner" for undercoat compound. Best pa rin yung 3M, though I am not sure ano yung Class B because AFAIK walang Class B item ang 3M and malamang binubullsh!t ka lang nung kausap mo and gagamitin nya one of those cheaper undercoat compounds in the market.

Another factor you guys should consider is how much kerosene vs. undercoat ang gamit, baka mamaya sobrang lipis din ang halo and it won't last you one flood. I got a painter to do our trucks here in our place, and 1 can of undercoat kaya 2 trucks, so siguro car underchassis around 1/4 to 1/3 of a can. A can of 3M costs around 1,200++ last time I bought one, while cheaper brands can go as low as 500 bucks. Just so you know.