My $0.02:Contact cement is "quick and dirty". You might get some slight discoloration if the yellow cement shows through, though not likely. I find that contact cement has about a 10 or 20 year lifespan; enough to get you far enough down the road that you won't get blamed. Use fresh glue.PVE glue is excellent and essentially permanent if the surfaces are clean, it works better if the key top is slightly roughened. Clamping can be a serious pain as it will shift or "creep" under a clamp. I use brass weights to hold it down; check alignment frequently. It can "bite" or set in as little as 10 or 15 minutes, though it takes hours to properly dry; overnight is best. One brand of PVE glue is Franklin Titebond Melamine Glue. The glue from Schaff is slightly better.I have heard many great things about CA glue and many swear by it. I have had to clean off countless bad CA glue bonds that came off (not mine), mostly because of poor preparation or glue "kicking" before the key top got on. I can't get it to work myself. Good luck!Duco Cement should work well, but I don't have much experience with it.I just repaired two pair of hiking boots with "Shoe Glue" that was very interesting. It is a type of hard silicone glue (vinegarish smell) and works well on my boots. There might be some new and useful glues coming down the pike someday.