As many people know, Porsches don't rust. Starting in 1978, Porsche hot dip galvanized the entire bodies of their cars. An expensive process. May people talk about the "value" of cars, and this is one of the reasons Porsches cost more. The extra effort pays off in longevity.
That having been said, Porsche, while being known for fantastic engineering, has also been known to make some significant boneheaded design mistakes. Some examples are: The "oil" fill door on the rear of the 1973 911, which many owners confused with a fuel fill door, and filled their crank case with gasoline. Or the mid-1970's 911 with the 2.7 L engine, which had an engine case that underwent thermal expansion faster than the head studs, thus making the studs pull out of the engine, and causing it to pull apart or "grenade" (a sickenly accurate term.) Then there was the early 944 water pump, which allowed the timing belt to pass my itself at less than 10mm going in opposite directions, leading to it hitting itself, the belt falling apart, and destroying the head. Or the timing belt itself, which "required" a prohibitively expensive tool to "tension" until 1987, when they built in an auto tensioner.
And then, there was this (see picture.) This is a rust free car, all over. But, there's a catch... Porsche engineers put the battery in a little compartment just in front of the windshield, right over the passenger side footwell. Batteries contain acid. The acid leaks in very small quantities. Rain or even water from washing the car washes the acid off the battery, and down under the battery. Add to the problem the fact that the drain holes usually get clogged with leaves and gunk and dirt, impeding the drainage. So, there it sits, where it slowly eats through paint, waterproofing, zinc from the galvanization process, and finally metal. This eventually causes a hole in the metal, and whenever it rains water runs down into the passenger side footwell. In the early 944s (up to the 1985.5 mid year change,) this is not a huge deal. It's a mess. It looks bad. It makes the car smell musty. But it doesn't have an immediate effect on the car RUNNING. Until, that is, 1985.5 rolled around, and some genius at Porsche thought it would be a great idea to reloacate the DME (the computer that controls the engine) to... You guessed it. The PASSENGER SIDE FOOTWELL.
So, we have... Battery. Battery contains sulfuric acid. Acid leaks. Water washes the acid away. Drain holes get plugged. Acid eats through paint, factory waterproofing, galvanization, and metal. Acid water leaks into passenger side footwell, and drips into DME computer. Computers + weak molar concentrations of sulfuric acid = computer that looks like someone was growing rock candy inside of it = non-running Porsche.
I always check this. On this car, the rust is already there, and is pretty advanced, but still water tight. So, I clean the area up, clean out the drains, and use a Dremel to expand the front drain hole (which they plugged in 85.5 models), apply POR-15 (a polymeric isocyanate that converts ferrous oxide to a non-reactive molecule, essentially making the rust into rust proofing,) then top coat with black asphalt repair compound (available at Home Depot in a squirty bottle.) After it's dry, I dust the area with calcium carbonate (baking soda), and reinstall the battery. From then on, when I change oil, I rinse the area, check the drains, and pour a little more baking soda just generally "down there." The baking soda is a base, and neutralizes the acid from the battery.
So, a little redesign and maintenance prevails.
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