How can I repair a rearview mirror that fell off the windshield and took the glass with it?

Question

Normally, if a rearview mirror falls off the windshield, it's simply a matter of buying a $5 adhesive at the auto parts store and calling it a day. Yesterday I hopped into my 2001 A4 (a secondary car for me) and I noticed the mirror was dangling from the windshield. I pulled it down and noticed that there was a chunk of glass glued to the button and a roughly 1/4" asymmetrical crease in the windshield where the mirror used to go (see below). Now I'm at a loss about what to do, short of buying a new windshield. This is kind of a hack, but I was thinking that maybe I could apply epoxy to the glass chunk and then apply the windshield adhesive to the rest of the exposed metal on the button. Unfortunately, that plan will have to wait a month or so until the weather warms up here in New York. Any other ideas? Is my idea sound?

Answer 1

I think the normal "rear view mirror" adhesive is not going to work, even in conjunction with epoxy.

The set time for epoxy probably requires some extensive clamping time, and I doubt just "holding it there" is going to work. Epoxy is too thick, and for the best bond you want a very thin layer and the maximum amount of surface contact.

You might give this a try:

Loctite Glass Superglue

I think this might work well, and have a fast cure (I beleve it to be cyanoacrylate based). It claims to be good for both glass-to-glass and metal-to-glass, so you're covered.

I have not used this product (although extremely satisfied with others from the same manufacturer) and I certainly have not attempted this particular repair.

However, I look at it this way: No matter what you decide, even a failed attempt really doesn't set you back much. The absolute worse case is a new windshield (and you still have to attach the rear view!)

I'd give it a shot. Can't get any worse.

The only caveat is you may experience crack growth from the depression as the temperature cycles over time. Automotive glass companies do have special materials that are injected into chips and cracks and then cured (sometimes with UV light) to prevent further crack growth, but I can't say if these work from the inside. It might be helpful to get a free consultation from a local auto glass company.

As an aside, I find this failure very interesting. I suspect it has to do with the different thermal expansion coefficients between the metal mirror pad and the glass. The adhesive didn't fail - I think the metal expanded with heat/cooling cycles and caused the glass to fracture over time.

Although I'm quite sure "interesting" is not the first adjective YOU thought of...

Good luck.