Roy Smalley: 356's started rusting the first time they got wet. The important issues for car owners to consider in protecting a 356 against rust include: 1) what are your expectations regarding future rust, 2) does one recognize there is no such thing as rust free 3) what can be done to minimize the future potential, and 4) the fact that the potential for rust in 356's cannot be eliminated.
I have had occasion to examine a few real restorations (as opposed to refurbish or tart up). These particular restorations were of 'apparent' rust free condition cars by the best restorers in the business within the past 5 years or so. The cars were beautiful from any standard. All had rust bubbles, some almost insignificant to not so insignificant. I won't say that I am in the best category, but one of those cars was restored by me. All were on the exterior skin where it was folded over the inner structure like door bottoms, fenders over inner skirts, hood and deck lid, from the inside out. All would be expensive to repair and get the car back to the unblemished level.
So the question is, how does one address rust you can't see, which is far more economically important now than a few years' ago. Door bottoms are an excellent example as they are the best water containment apparatus on the outer skin of a 356. The folded area was not protected when built, with bare metal folded over bare metal, or if there happened to be some finish before, it was perfunctory, not intended to protect from long term exposure to water, much less salty water. For decades, European Master Restorers have done complete skin off restorations, just because of this potential.
A suggestion would be to consider unfolding skins: doors, fenders / quarters at doors (although the quarter is less likely and of course, leaded), hoods and decklids (condensate on the inner structure accumulating in the void / folded areas). Unfortunately, some destruction is necessary to get to where you might need to look.
We just completed metal work on a one owner 61 Roadster. It was by all measure "rust free", with the exception of one little bitty pin hole in front of the left door. With the owner's agreement, we unfolded the fender joint at the door down to the rocker interface and you just have to see the deep pits on the inside of the fender to believe how rusty hidden areas become without exterior penetration. Both sides, because the original caulking had been upset, allowing water to penetrate high and no way to get out low, were very rusty. If we had painted the car without replacing metal on both sides, everyone would be unhappy. Just sanding the area without repair could have caused microscopic pin holes that would result in rust bubbles later on.
Bottom line, there is no guarantee. It might not rust "here" but rust "there" a few inches beyond what seemed "enough". So an owner has to look at what is realistic, select the option that makes the most sense for them on that specific car, do the best they can and live with the result, knowing that there are many ways rust can rear it's ugly head.
Bud Osbourne: You must eliminate the rust, unless you want to have to deal with it, again, in the (maybe not too distant) future. A lot of what we see advertised and advocated is touted as a "rust encapsulator" / "rust converter". Even POR-15's name comes from Paint Over Rust, which is NEVER a good idea. If you read, and follow POR-15's application instructions, carefully, you will find that you must remove virtually all rust, and treat the metal with an acid solution (to destroy the remaining rust), before you can successfully apply POR-15. And, if you read the instructions and recommendations, carefully; you will note that POR-15 doesn't adhere terribly well to "clean" (smooth), un-rusted steel. Applied to clean steel, which has been sand-swept, or sand-blasted, however, POR-15 is really rugged stuff.
There are various "goops" (including Ziebart, which is probably just fine for new cars, but not old ones) which tend to trap water / moisture in boxed sections, thereby speeding the rust process. This is certainly true, and such stuff is to be avoided. There are, however, "body cavity coatings" which have the following, essential properties:
1.) First and foremost, any body cavity coating should be ìwater displacingî. This means that water floats ON TOP OF IT, instead of the other way around. In other words, when applied to metal which has water/moisture on it, it will displace the moisture from the steel, and adhere directly to the steel. Any subsequent introduction of moisture (including total immersion) will not separate it from the steel.
2.) It should have excellent "creep" properties. This means that, in the case of the lower door edges of our 356s, where the outer skin folds over the flange of the inner structure, the body cavity coating should penetrate between the overlapping layers of steel, displace the moisture found there, and prevent future introduction of moisture on those surfaces.
3.) It should "kill" light, surface rust, which may have formed on interior surfaces during the course of restoration or reconstruction.
If you're looking for a "goop" to creep in and kill original, long-established rust, as found in our 356s; you will be very disappointed, as it has not yet been invented, in spite of claims of some snake oil salesmen. I would NOT recommend the introduction of Waxoyl (which is reported to incorporate the above listed, desirable properties) into an original, un-restored, boxed-in section of any 356. This is because it might tend to trap moisture in old, moderately scaly rust, which would prevent the moisture from evaporating (which it will eventually do, if the car is kept in a dry, well vented area); preserving the moisture, instead, to "feed" the rust.
Another caution for those tempted to use Waxoyl: the stuff REALLY creeps! In fact, it will probably creep a whole lot better/farther than you ever anticipated, and show up in places or on surfaces where you never expected it, nor wanted it to be. Hence, my mention of "lightly" coating the inner surfaces (probably just a squirt of it on the door bottom, against the outer skin, to allow it to penetrate the bottom lap).
So, there you have some clarification. Bottom line is, the only way to cure rust is to remove it / cut it out. And, NEVER try to cover over / encapsulate / convert rust. You will only be creating future headaches for yourself or future custodians of your 356.