I suspect you get what you pay for with rust converters. I have used Rust Mort by SEM and Krud cutter.
Rust Mort works much better on heavy rust than Krud Cutter but is twice as expensive at $95 a gallon than Krud Kutter.
Krud cutter is good at removing surface rust on bare sheet metal but doesn't penetrate very well.
Here is what SEM has to say. "Rust Mort is an acidic compound that converts existing rust to an insoluble black coating prior to applying body filler, primer or top coat. Completely removes light rust. Converts heavy rust into a stable, workable substrate. Creates ideal surface for filler or primer".