Tectyl 506 made by Valvoline. I haven't heard of using separate products for inside and out, but I know Tectyl does both jobs quite nicely.
My family has been using it for years on our 4-bys, and for obvious reasons we still use it.
I did mine 2 years ago - its one hell of a job if you want to do it properly! I stripped the vehicle down (short of pulling the engine out!) to get into the places salt can reach.
You can special order Tectyl from Supercheap in 5L and 20L drums - you need at bare minimum of 5L to do a vehicle. I have a dual cab, and we used about 8L.
This is how we do it anyway...
Use a hand spray bottle, and coat the underneath and all your panels paying particular attention to weld seems, bolts, joins etc. You will need to mix some petrol with the tectyl to thin it out making it easier to work with the squirt bottles, and to let it seep into those finer areas.
Just try and stay away from your exhaust - get it on there and you'll be smelling it burning off for at least 6 months! (I can still smell mine after 2 years!) No harm done, it just gets to your head after a while!
My cousin has made an air spray with fine tubing and a 360 degree nozzle to fit down chassis rails, through door sills to reach the weld seems, down pillars, and anywhere you cant reach with a squirt bottle. It works a treat and coats the areas.
Tectyl over time cures to a relatively hard wax, although you will still get sand and dirt sticking to it.
Depending how often you travel on the beach depends how often you respray. I go up a couple times a year, and I don't expect to redo it until after 5 years, but I guess if you spend more time on the beach, then perhaps its worth doing it more often.
I suggest doing the job 2 months before heading up the beach, as it takes a while time to harden to the wax. I guess doing in summer time would speed up that process.
When doing a full spray, use a chem suit or some old clothing (long), as the stuff can be pretty horrible stuff. It made me a bit sick a few days later - but perhaps that could have been dehydration, heat and lack of food since it took an entire day just to spray!