WARNING - Not a FIAT question!

I have an old mower I am working on that has a bit of rust in tight places. I've tried wire brushing as much as I can, but can't get to all the rust. What is the opinions for the best spray rust converter?

Try a phosphoric acid like Ospho. It's a liquid form so it will flow into nooks and crannies really well.
It'll convert the rust from iron oxide to iron phosphate effectively killing the rust.
Hit with a good rust paint after that..

I have tried several brands of rust converter and have not been impressed. In my experience they will only convert light surface rust.