What is the best undercoating to use after removing surface rust on the frame? I need a paint/rubberized coating and not rhino-liner/line-x.

 

Undercoating is NO GOOD unless it is put on from the factory. Otherwise too many contaminants and rust are under it and it will fail, causing MORE RUST

Do yourself a favor and look into POR 15, Chassis Saver, and Eastwood's Rust Encapsulator

 

what about rust reformer? would that work or would that be just the same?

 

Ive use that before with great results. I painted a toyota frame that had minor rust and then topcoated it with satin black-1 spot or rust came back. POR15 or Chassis saver is better of course and may hold up better overtime-but its also 10x as costly as 1 can of rustoleum rust reformer and 1 can of black.

 

well i got the rust reformer, primer and john deer green on the frame so its got triple layer to get through