During the course of my project build i have done a bit of research into rust removal and the options out there if you dont own a welder/sand blaster or grinder.
I am a big fan of Bilt Hamber products, if there is something that you can remove from your car then their Deox C powder os brilliant, you make up a solution by mixing it in water and submerse the part in it.
This is a rear diff i did a year or so ago.
Before
After
all i did was give it a light scrubbing with a wire brush, i just left it in the solution for about two weeks.
The gell they have is almost as good, but you can paint it onto parts that you cant submerse in the solution, i did my rusty boot lid this week and its come out looking great, no rust at all.
(and no Coke, vinegar etc wont do the same job....i tried it)
After that i have tried a couple of paints,at first i went with a highly recommended product called POR15 rust preventing paint, which i am not a great fan of to be honest, it has a great finish, very smooth and glossy, even when applied with a brush, it just dosnt stick to rust free metal very well, its good if you want to paint over rust, but thats not really the right way to do it.
After that i tried Epoxy Mastix 121 & Epoxy Mastic 421, both very good, the 121 is for the underside and chassis, the 421 is for body panels, both spray on with a gun, and both give a great finish on which to apply the next coat. expensive, but i highly recommend them both. (dont hand paint them with a brush as the finish is rubbish if you do) Not really rust killing paints, but they will help prevent it from coming back.
on top of that you can ut your desired stone guard, there are lots out there and a couple of very good ones, 3M & Soudal spring to mind, i have tried both and prefer the Soudal but they are both pretty much of a muchness. You can paint over the Soudal to match the colour of your car.
The next step of rust defence (which is the last thing you should do) is using a cavity wax. Now i know that loads of people recommend Waxol, but to be honest i hate the stuff, there is much better out there. i tried the winner of the car restorers magazine long term test Noxudol 700, it comes is a can and a canister to attach to a compressor, its quite thin so it leaches into the gaps between metal panels, after about 4 days it waxes up and remains in place. unlike waxol it dosnt harden up and crack.