I've used the Hammerite underbody seal that you mention with the added Waxoyl, after loads of filthy prep work and Granville rust treatment.
The cans with the right attachments are made for connecting a compressor air line for general all over spraying and getting into cavities/chassis rails.
Proper prep is essential, or results will be a waste of time/money, and own efforts often surpass some of the so called 'professionals' that just cover everything up
As Karl says, everyone has different ideas on how to go about it and products to use.
There are, as was mentioned, loads of threads/info on the subject, - a lot on the 120 forum, where rust can be an issue.
I know, I have one, although rust is not an issue now as some was caught and dealt with by some cutting and welding replacement inner and outer cills, at at my local classic car workshop, (more experienced/sympathetic than a general welder who doesn't normally do paint) the rest for onward treatment by myself without issues.
God luck on whatever you decide, and unless you are under cover, a job for better weather.