Hi and welcome to TMF.
There are a plethora of products out there that can help you BUT like any of them its all down to preparation.
My choice would be Dinitrol and their various products. (something we use in Civil Aviation.) Bilt Hamber is another good product. Waxoyl is still good as well. You will hear detractors but its easy to apply.
Now to the crux of the matter. If your car has had no welding or is not rusty at all???? then it has lead a very sheltered life or the previous owner was fastidious about maintaining it.
Yes they do exist cars like this but knowing from personal experience they are rare to be totally rust free. However i digress, so the best thing to do is get the car on a set of ramps / four poster etc to have a really good look for anything at all. Primarily you are looking at the underfloor in total, heelboard, boot floor etc etc. Look for anything flaking or lifting at all, dont just brush over it. Now if all of this is solid your next job it to properly clean it, get up into all of the nooks and crannies and then dry it off (dont be tempted to apply anything even if it says it can be applied over wet/damp areas.)
So baring in mind now that this process is for a car that needs no work other than applying a wax based protection product. Depending on what you purchase you need to make sure its applied to all exposed areas and also right into all of the little corners and areas where muck will accumulate. Be patient and particular in how you do this to get the best protection (no spray and pray application) Once done, clean off any excess.
Now there are areas like the following where your particular product will need to be injected. Inner sills, A and B posts, front bulkhead crossmember, rear valance closeouts, seat crossmember, the rear subframe. You can access all of these areas although the bulkhead crossmember is the the most difficult and will need some lateral thinking on the best approach (front footwells, tucked up under the lower dash rails there is a cutout one each side.)
The inner /outer sill can be accessed via the flutes in the outer sill. Be very liberal with your application here to ensure you get the best protection possible. The rear valance closeouts normally have drain holes in them so use these but make sure once done they are clear to allow any moisture to drain out. The A and B posts are best done from inside the car and may need some trim items to be removed. The floor crossmember has a few holes that can be used to pump your product into.
The rear subframe. this is notorious for collecting detritus so it really needs to be thoroughly cleaned. Once done liberally apply your product into every gap you can see. The front subframes are normally very easy to access and what you see is what you get. (engine oil from any leaks helps to maintain them)
Doors, boot lid and bonnet. The doors will need coating inside, so take off the door cards, carefully remove the plastic liner enough to get access then pump in the product. Do make sure the drain holes are kept clear. Reassemble everything ensuring the plastic membrane is fully sealed on. This is important.
The boot lid is easy to do when its opened. The bonnet depending on whether its a roundnose (which your car will be ) or a clubby will need some careful application into the underside bracing.
So for a straight fwd on the road car process, then that is pretty much it.
So next (yes there is more and it depends on the underside finish) The underside as it currently is. Is it painted?? is it stone chipped?? Is it covered in wax based product??
If its painted and i mean by painted onto metal and nothing else then the wax based products are what you need. But obviously you need to ensure there is no bare metal / damaged paint at all.
If its stone chipped (so a mottled rough looking rubbery finish) a couple of things here to think about. If any of it is flaking at all then that affected area needs to be removed (a wire cup on a grinder will do the job) until its all solid. You can buy spray cans of stone chip to re apply to those affected areas. Apply a topcoat of paint after if has been painted. Now you can apply a light clear coat of a wax based product to all of it for extra protection.
If its covered in a dark / black wax based coating, this makes checking anything underneath impossible to really determine its state. But if you are happy there is nothing dubious lurking there, any areas where its missing, thinned down, just apply some more.
Quite a lot to think about but its in the cars best interest to make sure there is as much protection as possible. Giving the car regular washes especially underneath paying particular attention to the front inner wings right up under the scuttle corners, the rear subframe and so on.
Ideally when at home the car needs to be under cover if possible, but if not you can get external covers to help protect your pride and joy.
