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Repairing a broken mirror housing

Forum member Carson requested help and suggestions with repairing the broken plastic housing of a mirror on his coach. Forum member Frank provided some expert technical help. The following was extracted from the discussion.

If the original broken piece is not available, use a similar plastic for the repair. Another broken mirror taken from a vehicle in a scrap yard, or obtained from a replaced and discarded mirror in a body shop, would be ideal. Alternatively, ABS plastic from plumbing pipes or fittings can be used for the repair.

Take a pair of tin snips, and cut out a chunk that approximately matches the piece you want to replace. If the broken area has a ridge, you can save yourself a lot of hand work afterward if you're able to find a piece that already has the 'ridge' molded into it.

You will want to visit NAPA or some other commerical autobody outlet and get a tube of Duramix 4040 (the strong glue) and a tube of Duramix 4058 (the finishing glue). These are semi-rigid repair products, and can be had in small (2 ounce or so) dual-tube packages with a plunger in the end. Epoxy won't stick to plastic, and epoxy also is way too hard when it sets. Vibration or any other 'knock' on the repaired area afterward will break it loose.

Carson found an alternative to the Duramix; Easy Finish by Bondo-Mar/Hyde Corp is a Flexible type of 2-part epoxy resin in small tubes, and is used specifically for flexible bumper repair. It works as an adhesive and also a filler. Can be sanded to shape. Color is black.

Before you begin, rough-up the inside surface of the mirror housing around the break. You can use coarse sandpaper for this. 100 grit is what we use. The rougher, the better. Then clean it with a bit of lacquer thinner on a clean rag. Don't use excessive amounts of the lacquer thinner as it will soften the plastic and melt it if you use too much. Don't let any drip down the side of the coach either for the same reason. If you get some where you don't want it, just wipe it off. It is quite volatile.

Cut your plastic to fit the hole as best you can, and rough up the inside surface that will mate with the mirror housing the same way you did inside the mirror housing. Within reason, the more effort you spend shaping this replacement part, the less trouble it will be to work with later. Then use a soldering gun or soldering pencil to 'stitch' the replacement plastic into the housing of the mirror, melting the two parts together. Tack one end in place, then do the other end, then work from the middle out toward the two ends when stitching. You are less likely to end up with it mis-aligned that way. Don't overlap the parts, but rather make a butt joint. Don't worry if the seam is not neat. It just has to hold it in place while you work with the 4040.

Once you have your plastic 'patch' in place you are ready to use the Duramix 4040 plastic repair kit. If you got the small tube, it usually has a small piece of fiberglass 'screen' in the kit. If not, you can get drywall fiberglass tape from a building supply house. We use the drywall tape as it is sticky on one side, and holds itself in place on the part you are repairing.

If the plastic patch it not thick enough, then try to stitch it in place as close to flush with the outside of the mirror shell as you can. It will save you some work later. If the inside is not 'level', you can either just leave it that way, or fill it with a second coat of the 4040. The 4040 only needs to go across the joint, however. Use the 4058 as filler afterward if needed.

This is an approximate thing, so don't worry overly about the placement of the fiberglass tape. You just don't want to put it and the glue where it will bind against the mirror when you adjust the mirror. If you have room, you can even put some of the fiberglass tape over the ribbing inside the mirror housing, and put a bit of glue on them as well. No one is going to see the inside of the mirror. Or, if you want (and if the ribs are thick enough), you can even drill a small pilot hole in the end of the ribs, and push a short piece of copper wire in there. That can then be glued to the inside surface of the patch, and will work like re-bar.

Move the mirror as much out of the way as you can by either moving it in or out until you can access as much of the inside of the area you will be repairing as possible. (If you can remove the broken outer housing altogether and work on it apart from the rest of the mirror, this is even better.) Stick the fiberglass tape over the seam. Don't worry if it hangs over anywhere. Just make sure that you get a single piece of the tape more or less equally across each side of the break. You can cut multiple small pieces of the fiberglass tape and place them where needed around a curve. Try not to overlap the fiberglass tape excessively, as it will just prevent the glue from reaching through to the surface of the mirror housing. A single thickness of fiberglass tape is strong enough.

The Duramix 4040 sets very rapidly. The kit you have will come with some mixing 'barrels' that have a bayonet lock on the end of the tube. We don't use them often as the product sets so quickly. I just cut the end off the glue tube, and then squeeze some of the product out on a piece of cardboard. There is a yellowish clear liquid, and a black liquid. Stir them together with a popsicle stick until they are well mixed, then use the popsicle stick to 'paint' the glue over the fiberglass tape you applied inside the mirror. Squeeze it down through the fiberglass so that it makes as much contact with the plastic under that tape as possible. You want to apply it thick enough to cover the fiberglass tape, but there is no need to make it much thicker than that. The 4040 is extremely strong on its own, yet remains somewhat flexible. An amazing product that really sticks well. Remember, however, that you will only have a minute or two to do the applying before it starts to thicken. It will be set in 10 minutes or so. If you didn't mix enough for the whole job the first time, you can always mix more later and apply it over the remaining surface you may have missed. It also sticks to itself very well.

Once hard, you can use the 100 grit sandpaper again to smooth out the inside surface, and scrape away any of the fiberglass tape that didn't get covered with glue. If you think you need it, and you have enough clearance, you can wipe it clean with lacquer thinner on a cloth, then apply a second coat. This repair should now be very strong. If not, then let me know. There is another step we can take, if needed, on the outside of the mirror housing.

If you get any of the 4040 on your hands, it will wipe off with lacquer thinner while still soft. Once hard, you'll have to wait until it wears off, which can take several days.

For the outside, sand off any 'lumps' that you left in the plastic when melting it together, clean it well with lacquer thinner, then mix a small amount of the 4058 on a piece of cardboard as well, and smear it over the break with a popsicle stick, trying to squeeze it into any holes that are in the surface of the crack. The more you apply, the more you will have to sand later, so don't go crazy with it. However, with that said, you can use the 4058 to fill and shape the outer shell as well. This product also sets quickly, but not as fast as the 4040. Let it set well before sanding it, probably starting with 220 dry paper to shape it, and then finishing with 400 or 600 wet paper.

The 4058 sands well, and will accept paint directly, though I believe they recommend that it be primed first. We use matte vinyl paint as it sticks well to the well-cleaned plastic on the mirror housing, but you may not be able to get any. See what NAPA can offer you in a spray can.

Carson's additional creativity

Carson decided to make an accurate template on a piece of filefolder stock. However, there was no way he could get a pencil on the inside to trace the irregular outline of the break, and to do it from the outside would be hit and miss. He borrowed a red lipstick from his wife and put a layer right on the edge of the break on the outside. He then placed the paper stock over top and got a precise imprint. A pair of scissors trimmed the stock to a perfect template.

Carson kindly provided photos of his repair following the above procedure.

Before repair

First fit

Ready for paint

Completed job