Not sure if this is better suited in a general forum but there might be some specific points relevant to a Focus MK1.

I'm learning about how to go about protecting the underbody from rust.

When it comes to a rust converter, Jenolite Rust Converter Spray 400ml seems to top every comparison test I've googled. That said, Dinitrol RC900 seems even better but it's never been compared. It's over twice as expensive as Jenolite as well.

Has anyone sprayed the underbody with a rust converter and if so, would one can be enough? I think more than one would be needed which makes the Dinitrol too expensive, unless anyone believes it's worth it?

After the rust conversion, I'll need to find the best protector. It seems Lanoguard or Waxoyl are possibly the most well known but are there better ones?

Thanks very much and hope you can help.

 

Don't know about the rust converters and how a topcoat wold react with them.
IMHO Waxoyl is a real pain, you (I) have to dilute it down and keep it hot and even then, any spray bottles clog up, it's very messy.
I bought Lanoguard and the grease seems really useful for exposed nuts and places where you want extra protection.
The topcoat sprayed on REALLY easily with no hassle at all.
The downside it it does feel rather expensive and there appear to be cheaper versions on eBay (which I may try in future - if it lasts that long)
They suggest 2 coats for older/more rusted vehicles and then re-treat every year (but it will be a quick and relatively clean job compared to my multiple  bad Waxoyl experiences)

Interested to hear other experiences!!