Well it's finally nice and warm and all the convertibles are coming out. So let's discuss the topic of convertible tops and their plastic windows. First let's start with the top itself; they're either made out of vinyl or fabric. Since all materials eventually fade, manufacturers don't make specific claims against fading. In case you've had your top replaced, it's also advisable not to lower it for at least 1 week after installation because the top needs
time to stretch and conform to the shape of the frame.
Now let's go over some of the rules of thumb for maintaining your convertible top:

    *      After enjoying a day in the sun, always raise your top prior to storing your vehicle (if you're unsure whether to leave the top down or put it up, then put it up). Leaving your top down for more than 24 hours gives the material time to deform and may produce stubborn wrinkles. It's
      like throwing a nice Armani with crumbled up in your trunk and not expecting it not to be wrinkled the next day.

    *      Do not lower your top when the temperature is below 60° F (even though many car manuals say 50°). Doing so may crease or worse yet crack the vinyl window; and yes I've seen windows crack right down the middle on more than one occasion.

    *      To keep stubborn zippers working smoothly, try rubbing a little bit of solid hand soap or candle wax on them.

    *      Never wipe the vinyl rear windows dry nor use paper towels (most paper towels are made fromwood and may cause scratches). There are soft disposable towels made expressly for this purpose.  I am not a fan of using a soft old t-shirts or towels either because if it has even one particle of dirt in it and you wipe your window with it, OOPS! And by the way, don't ever use an ice scraper (don't laugh: I recently spent an hour machine polishing a convertible window that was "cleaned" by one.

    *      When folding your top down, always place a clean cotton cloth/towel or baby blanket between the folds of the vinyl window to help eliminate scratches and rub marks as well as eliminate a sharp fold in the plastic. There is a product specifically made to address this issue called the "window pillow" that I highly recommend. Remember that as you drive your BMW, it vibrates if only ever so slightly. Thus, if any part of the vinyl window contacts another part. It will result in a dulled rubbed area. You usually see a set of four these moon shaper marks on most convertible windows.

    *      When washing your plastic convertible window (just like washing your car) it is important to rinse the surface very thoroughly to remove as much dirt as possible. Particles left on the surface will act like an abrasive once you start to wash. I suggest using a natural wool sponge with plenty of car wash soap and using no downward pressure. The best way to dry it is with compressed air (because you're not touching the surface). If not use a synthetic chamois and pat dry (do not wipe). The remaining dried water residue can be removed with "Novus #1" or Plexus with a special disposable towel as aforementioned.

If you are going to polish your plastic window, do so after washing the window and before washing the rest of the car. This is because the good polishes are water based and in case they get on the fabric top, they can be rinsed off with a strong stream of water (my favorite brand of polishes is "Novus"). Now, BMW does you a big favor and installs a plastic border around the window. After polishing, the window should be sealed with
"Plexus" (in which case you have to deal with its slight greasiness) or a quality carnauba wax (just be careful to not get the wax on the fabric).

If you're waxing your BMW convertible, take caution not to get polish/wax on the edges of the fabric top. I would suggest taping the edges with some blue 3M "Long Mask" painters tape (basically masking tape which leaves less glue residue). If you do get it on the fabric, on boy!

As you've probably noticed, brand new fabric convertible tops are coated with a water-repellant (the water beads ups on the fabric). As that coating wears away, you may find the top starts to wick (absorb water from the lining side) and thus require more cleaning. This is the time to reapply the coating (this should be done on a regular basis; just think of it as a routine like waxing your paint). The top should ideally come clean with a good stream of water. In most cases, I try not to physically touch the ragtops when I work on them. Unless there is a particular stain, do not use any kind of cleaner or rub the fabric. Every time you touch the top, it's like rubbing a sweater; you create lint and wear away the fabric.