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How To Fix A Leaky Windshield


How To Fix A Leaky Windshield


I've since done this procedure again, this time on the front windshield of my '72 280SE 4.5, and it worked great. In short, you can reseal it yourself, but the DIY materials at the parts store or home depot are not the right ones to use. If the gasket is in good shape (not hard and cracked), I'd attempt to reseal it in place long before I'd have it pulled out and re-gasketed, unless the window is cracked and needs to be replaced anyway. There's really no guarantee of a leak free window even when you use a new gasket.

Finally time to reseal my rear window. At one time, I (unwisely) tried to seal temporarily with silicone. It didn't work, but the silicone did adhere to the paint. Anyone have a good suggestion on how remove it?

TIA,

Sam Williams

Sam -
Finally, a question to which I can lend some actual expertise. If the silicone is bonded to the metal, leave it there. One of the nice things about silicone sealant is that new silicone sealant will bond very tenaciously to existing silicone sealant. If you got it to stick to the metal, you've won half the battle.

Here's what I recommend:

Use a razor blade to carefully cut away most of the existing sealant, leaving a thin film adhered to the metal. I'm assuming that it did not adhere to the gasket. Do the same thing at the glass too, if you sealed the gasket to the window.

You should not seal the window up completely. Seal the glass-to-gasket interface all the way around, but seal the metal-to-gasket interface only at the top and sides of the window, and around the bottom corners. Leave the bottom (sill) of the window unsealed - you're unlikely to get any water leaking in there, unless the gasket is in REALLY bad shape, and it allows escape of any moisture (liquid or vapor) that may find it's way into the gasket. If you seal it completely, IMO you're much more likely to experience the dreaded corrosion of the steel window support flange at the bottom corners of the window. Also, try to perform the work after a long period of dry weather, to minimize the amount of moisture that may get trapped in.

Carefully clean the metal surfaces beneath the gasket, the window surfaces beneath the gasket, and the inner surfaces of the gasket. Use what's called the "two-rag" method, with clean white cotton rags and isopropyl alcohol. Wet a clean rag with alcohol, and run it back and forth between the gasket and metal, or gasket and glass. Immediately run a dry rag back and forth in the same spot, to remove the alcohol before it evaporates. Do this in short areas, repeating the process with clean portions of the rags, until the rags are no longer removing dirt and debris. This will be a tedious process, as the gasket is bound to have lots of loose surface junk that has to be removed. It's relatively easy at the metal-to-gasket interface, as the outer lip of the gasket is thin and flexible. The glass-to-gasket interface is tougher, you may need a popsicle stick or something like that to push the rag under the thicker gasket lip.

Mask the metal and glass surfaces adjacent to the gasket, as you don't want the silicone to get on the visible surfaces - as you've seen, it's hard to get off!

Now, it's ready for new sealant. Silicone sealants are generally classified as either acid-curing or moisture-curing. Acid-curing sealants are more common to consumers - most people acquaint silicone sealants with a vinegar odor. However, these materials are low-performance - they may work for awhile in your shower stall, but they're not going to provide satisfactory results for the temperature extremes that a car sees. What you want is a commercial-grade moisture-curing silicone sealant, in a low to medium modulus of elasticity, such as Dow Corning 790 or 795 or GE Silpruf. Your best bet would be to try to buy a tube from a local building window contractor, as the real distributors of it probably won't sell it retail or in quantities of less than a case. As far as color goes, get black, don't ask for "clear" because it doesn't come that way. I recommend that you not use the "windshield sealant" that's sold at the parts store, it's a very low viscosity material that will flow into all the spots that you don't want it to flow, and none of the spots where you really want it.

Keep in mind that silicone sealant has a very finite shelf life - it must be used within one year of manufacture, and preferably within six months. If outdated sealant is used, adhesion will be greatly compromised, and other problems such as streaking of the glass and staining of adjacent surfaces is common. This is another reason not to use "hardware store" sealant - commercial sealant has a "born-on" date stamped on the tube, but hardware store caulking generally doesn't.

Cut the tip of the sealant tube to lay a small bead of sealant, about 1/8" or so. Stick the tip under the gasket lip, and lay a small bead of sealant between the gasket and the metal or glass. Lay the gasket flap down in the sealant, and press lightly to seat it. Don't pump the groove around the window full of sealant, just use enough to seal the gasket flap to the metal or glass. More is not better. Some should ooze out onto the masking tape. Let the sealant skin over, and then pull the masking tape. If any gets on the paint, window, or visible surfaces of the gasket, remove it right away with clean rags and alcohol. If you allow it to cure, it will be difficult to remove, and it will attract dirt, which will cause it to be very visible. Ideally, let it cure for about seven days before driving the car. Realistically, try to keep it under cover (out of sunlight and rain) for at least 24 to 48 hours.

Some have recommended 3M adhesive remover - while this is great (nasty) stuff, I'd be surprised if it would even touch cured silicone.

BTW, I've been down this road before. My 240D window leaked for several years, so I finally had it removed and re-installed with a new gasket. The guy did an *OK* job, meaning that it got done, but I'd never go back to him. It began leaking again within a year or so. So I resealed the window as described above, and have had no problems since.

Karl Strand

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