The vent tube is for the hydrogen gas escaping from the cells as they charge, not for excess acid. Seems you could safely connect it to the vapor system?

Quote.... "The vent tube is for the hydrogen gas escaping from the cells as they charge, not for excess acid. Seems you could safely connect it to the vapor system?"

Interesting...
When I bought the car the battery was resting in a plastic tray. Inside the tray was a clear liquid (about 1/2-inch or less) that I assumed was acid. Not being the dumbest thing walking I didn't taste it to be sure it wasn't water! I just assumed it was acid.
The battery is only a few months old, so it makes me wonder if acid had indeed ran into the plastic tray via the vent like I suspected.

After getting into the box I have found that most of what you see in the photo is discoloration of the rubberized coating on the bottom of the box. There were only a few places where this coating was broken and the metal exposed.

Last night I posted on the paint and body forum about my intention of stripping the entire box to bare metal and doing the POR-15 thing, but now I'm wondering if I should consider not stripping off all that coating as most of it is still afixed strongly to the box. My thought is that since there is a small opening of the metal (top left corner in photo) maybe I shouldn't seal it up and POR-15 the whole thing before maybe letting a qualified body man have a look at the underside of the box to see if there's hidden corrosion. On shining a light, the hole in the box not is visible through to the underside of the car, but what if, right?

I hate not doing the entire job I've started but in light of the work involved to do it and possibly only having to cut away some of the metal to check underneath the box, how would you guys proceed?

Any opinons?

Thanks!

There will likely be some (hopefully small) areas where the PVC coating has some corrosion happening. These areas must be cleaned to bare metal. The trouble is identifying any such areas prior to getting busy with a wire wheel or such. Some areas will be obvious, some less so. Where to draw the line is a hard choice. Each step you take gets you that much closer to just ripping the whole thing apart and re-doing it! Looking at the picture, things are quite advanced on one spot along the lap seam where the fuel tank support meets the longitudinal. There is a good chance that acid worked its way through that seam. That is very similar to how mind looked. But again, don’t take that to mean that it is as bad as mine was – there are likely many factors at play.

Based on my experience as well as feedback from others:

1) The chance of there being corrosion in the void between the suspension pan and fuel tank support is reasonably likely. The tray is not perfectly sealed and usually a little acid gets in there when there has been a leak, and it doesn’t take much… This is something that lurks on a lot more SC’s and Carrera’s than maybe we want to believe. I had thought I had very little corrosion and that it was mostly just stains (that wiped off easy enough). Boy, was I wrong…

2) Having said that, it quite possibly is not MUCH corrosion in your case, so the real question is how perfectly do you want to fix it? Mine was not rusted through, but the sheet metal was noticeably thin in some areas. You may want to investigate this. Underneath the car, push VERY firmly with your thumb EVERYWHERE on that portion of the suspension pan between (and around) the forward lower control arm mounts and along the area in front of the tank. I was able to visibly see the sheet metal deflect in a few areas where it had become quite thin. If your car passes this test, then it would mean that if there is any corrosion in that void, it has likely not gone too far. In this case you may elect a fairly non-invasive approach (described below).

3) If it fails this test (or worse, if you find areas of rust-through) then I think the only real option is to replace the sheet metal.

In summary, the fundamental problem is that the most important damage is the least accessible. Soaking everything you can in POR 15 or such may not get to the crux of it.

Looking from the underside up into the void is not what you want, you need to look from the top-down. All the corrosion (if any) will be on the bottom surfaces of the void.

“Non-invasive” approach (where corrosion damage is minimal):

1) Remove fuel tank (you will want to make sure that acid did not soak into the fuel tank seal and start doing its nasty work there). Not difficult to fix if it did. However, you will need a new fuel tank seal to re-install, no matter what.

2) Strip all the PVC from the battery box area near any corrosion. You will figure out what you need to remove as you go. If you use a brass wire wheel on a drill, with moderate pressure, you should be able to remove the PVC and leave the E-coat still on the steel (well, my ’85 had E-coat – other model years may vary on that…)

3) Grind/sand away all corrosion. The brass wire wheel alone may not fully eradicate the corrosion (especially if there is any pitting).

4) Prime and paint. I know people are big on POR-15, but I would prefer a series of activated auto-refinish products from a quality supplier (BASF, PPG, DuPont etc.): Etch primer, stoneguard/pvc of some type, HS primer and then top coats (base/clear or single stage) would allow you to get a very OEM look – however, it would be more expensive and complicated, so it is up to you.

5) Using an undercoating wand, liberally spray the inside of the suspension pan/fuel tank support void with a wax-based corrosion protector such as FluidFilm. If you don’t have an undercoating gun (not too expensive) FluidFilm is available as an aerosol and if you use one of those plastic “needle” tips and spray liberally in all directions, you should be able to do a good job (and undercoating wand is just more “definite”). This should arrest and rust that is happening and prevent things from getting worse. You will spray through the three vent holes on the underside.

It all comes down to how much of a perfectionist you are, how much damage there actually is, and how long you are willing to lay your car up for. If you ultimately do go the suspension pan replacement route, it is a do-able job, but it will take a while!

P.S., I surfed over to the paint and body forum, I guess my stories got you worried… Sorry about that, I was just trying to relate some of what I learned. I always prefer to know what the real deal is. However, I do apologize if I have caused you undue worry.

I think if you do the thumb test I described above, and just generally knock around on the sheet metal under the car, it will hopefully assuage your worst fears. My fingers are crossed for you.

Thank you so much for the great reply. Worried a bit, yes, because "she" is my baby now and I want her to be around a very long time... and, I don't want any possible corrosion to worsen to the point where we have to do major surgery!
Thanks again!