Great thread for the unaware.
Interesting as I am sure everyone wants to protect, what for some is their pride and joys.
I treated and protected mine as soon as I got it, same as i have all of my previous ones.
It took me a good few weeks to do properly ensuring the car was dry stored first, ensuring most if any moisture was eradicated before starting.
I treated any areas that required it, used primer to cover any treated areas, top coated, spray on underseal in difficult to reach places, covered wheel arches, inner wings, all underside, inner and outer sills, rubberised stone chip on lips.
I still got the back end to do under the fuel tank.
I used.
Shutez spray on underseal (spray)
Tetra Shutez stone chip (spray)
Waxoyl underseal with added waxoyl (brush)
I plan to spray thinned waxoyl as a flexible covering layer come summer time, which should finish it off and be easy come top up time.
I have no doubt it won't stop anything that's already got in (that you can't see), but will be extremely effective in combating/ preventing future issues hopefully.
It's hard work and takes a lot of time, but has given me peace of mind.
I don't tend to use my car in wet weather and also dry store it, but if I need to I'm happy in the knowledge I've done some rust prevention.
Nb. I would be interested to hear what other people have done, as I just did my best with research personal experience.