i have not used it as undercoat, but I thought this might help: I installed a new brake system on my 77 f250 this winter, including a new master cylinder. I finished the install of the master cylinder at night. The truck is stored under cover, but living in the Pacific Northwest, winters are rather damp. I made the mistake of not painting the master cylinder before install. Woke up the next morning to finish a few things and I was horrified to see my brand new master cylinder with quite a bit of surface rust on it. The weather was just above freezing, so painting at this point was not an option. I used a rag and sprayed/wipe a number of coats of fluid film on the master cylinder. I haven’t touched it in 5 months. The master cylinder looks brand new! I am planning on degreasing/cleaning the fluid film off and painting the master cylinder. But for rust prevention on my master cylinder, this product worked awesome!

good luck!!