There has been some unnecessarily rude comments made in this thread.
My two cents:
First POR-15, Zero Rust, Encapsulator, all 1K moisture cured urethanes.
They are not chemical treatments. Stop calling them that! They are not rust converters as they are often called. That is a completely different product. They are a paint like coating. They do not chemically react with rust. They bond to it.
Moisture cured urethanes are used to paint off- shore oil wells and bridges. No question. The brands above are just marketed to retail consumers - not commercial industry.
POR-15 and Encapsulator must for proper adhesion be used on rusted metal. No question. They will only work if used correctly. POR-15 requires pre- treatment for use on smooth metal - and I have heard of failures using it on smooth metal even using the pre-treatment.
Encapsulator is not recommend for use at all on smooth metal.
I don't see why pre-rusting is a bad idea if you are going to use them. They need something to bite too. Kinda' scary I admit.
If rust is present and cannot be removed I use them. If rust is not present then I would choose epoxy instead.
Of the three Zero Rust adheres well to smooth metal but takes several weeks to fully cure and can be very soft until fully cured. Works well when cured.
Here is a good read that explains more: