There is a thread a couple months back on the subject.
 We have been thru this recently with our Big horn.

The bighorn had never seen the beach.. clean as underneath doors etc. Mechanically pump injectors , glow plugs and few other things replaced /updated regular services only done 200K
 Market value would be under 5K
Will get replaced but not worth taking the trade in value.. so keep as a beach hack...
But underseal 1st.
We shopped around a few panel beaters.. recommended and other wise..cost between $350 and $500... drop off pick up next day.
Smudge being local advised us to go chat to a local engineering firm that does vehicles for local mines and contractors.
An eye opening chat..
 They take the vehicle on a friday remove panels, fuel tanks and stuff to steam/ water blast on hoist, clean seams etc.
 Then let dry out over the weekend.
 Then seal everything up , spray in panels crevices, and finally under seal...cost  ball park around $800/ 900.

As they have normal contact/ industral work on , bread and butter customers.. and sealing is a minor side line, we arranged to have it done 'between jobs' as time available which happened after about 3 weeks. They had the big horn from a Friday morning thru to  end of Tuesday.

The thing with undersealing rust proofing.. it should be done with the new vechilce and never been driven on a wet road for best results.
Anything in seams, from moisture to road grit or sand, espec black sand, if ever been over road works..under sealing traps in

my-new-to-me toyota
 Son is on his 3rd hulux, he does big ks sees some serious country and trades for a new just before 100k every 3 or so yrs.
So doesnt under seal. the 1st the seams had serious rust starting and the 2nd, only a few months back not as bad but was there.

Bounty hunter.

Are you stripping the body to pan.
Do you have deep,pockets.

I say that as solutions have different outcomes.
From raw with the proper techniques you have endless longer lasting options. But they cost.

Knowing your use helps.
And any system will require maintenance more so in harsh environments such as salt, mud bashing fresh water environments and outdoor uncovered storage.

There are wholesale brand new pretreatment formulas.

To managing existing vehicles with unknown history.

I can help give you a path if you wish to provide more info on your needs.

ended giving the job to a chap in Manukau - underbody steam clean - and then a few touch ups with converter/sealant on some of the running gear - his comment was that the condition was remarkably good for a 2007 vehicle - and then a full spray underneath including inside rails with a wax-type product from DINITROL

Did they do inside the doors and body panels?

Yep..
 Older doors have older window seals.. salt beach spray runing down inside..
 These are easy enough to pull the panels inside the doors and a good  brush /vaccum out...squirt with a aresol of lanocane or fisholene, or that tecryl stuff.. and make sure the drain holes are clear.

Wash well up between hinges and pillars and door catches are .. let dry and cover them as well.
 If can around pillars , window screens
 All places that get hit hard by WoF.

And may lift the caropets in the trunk,  cavity down behind the wheel wells