Hi Ed,
Road salt can do a lot of damage. But, I'm not sure undercoating is a complete or necessarilly good solution, either. Especially not the stuff you buy at the auto parts stores in spray cans.
Waxoyl and similar products are good, but not generally used on large, exposed panels. Instead, it's sprayed into rocker panels, frame rails, etc. It was also used by the factory to coat moving parts that need some protection, like the handbrake mechanism.
Someone else mentioned undercoating bubbling or peeling, and I can show you where that happened on my car. Under those loose areas is where problems can really get started! Road dirt and salt get back there and stay damp longer, making a field day for rust and corrosion.
I'd suggest taking the car to a professional for proper undercoating. They should steam clean/pressure wash it first and then apply a coating. It's often not the ugly, thick black stuff that used to be common. Be sure to get the hidden areas treated as well, such as inside rockers and frame rails. This means drilling a hole to spray something like Waxoyl inside, then plugging up the hole. Most rust occurs on these cars from the inside out.
My TR4 was driven year round through Colorado winters and salted/sanded roads for many years, about 120,000 miles total, then stored for many more years. I am currently doing a frame-off restoration and have been surprised how few rust problems I've actually had to deal with. I attribute this to one thing, the open crankcase breather circuit on the older cars. It pretty well coated the bottom of the car with oil and kept it fresh for all the years it was on the road (1964 to 1986). Removing the gearbox, I found a half inch of oily coating. But once that was cleaned off the box looks like new, even the black oxide coated bolts are rust free.
On the other hand, the rockers on this car were replaced with fiberglass in 1979, due to rust. They are too far outboard to benefit from the oil bath provided by the engine! At the time they were replaced, all fenders were removed, stipped to bare metal and coated heavily with Rustoleum or something similar on the hidden surfaces. It still looks like new in there!
The problem areas are related more to poor sealing, such as under the polished aluminum capping used on the rear cockpit rim. Foam-like material used there acted more as a sponge than a seal and caused rust to occur right through 20 coats of lacquer paint, a sealer coat and several coats of surfacer/primer. Another problem area is the base of the B-post, which wasn't adequately sealed from the spray inside the wheel well. Live and learn!
Look for areas where road dirt is builidng up and trapped, such as the lower corners of the fenders. I think that "mud" is more damaging than road salt, but made even worse in combination with salt!
I don't know about the TR6, but they give very good sealing instruction in the TR4/4A factory manual (many of which would apply to TR6, too). I regret I didn't have that manual when I did the rolling restoration in approx. 1979. I could have avoided many of the problems I see now!
Hopefully these will give you some ideas. I don't think winter and salt will ever keep my cars off the road! There is much that can be done to prevent problems, not just quality undercoating.
Cheers!
Alan
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