Incidentally, DEET is not all that nasty. I just looked into this two years ago and am trying to remember some of the details. It has been used for close to 50 years now by millions of people. I believe it has been formally studied by the FDA twice. It has also been studied by numerous other sources and as of two years ago nothing substantial was found. I seem to remember that a small number of people may be allergic or sensitive to it. Just don't drink it!

The damage to the plastic watch was caused by a solvent, and probably the solvent used to disperse the DEET. Remember a can of insect repellent contains up to 70% inert ingredients, which may be inert to insects but not to watches.

Also, it is actually the vapor from the DEET evaporating from your skin that repels insects. At a concentration of about 25% the air around your skin becomes saturated with the DEET so using more concentrated DEET does nothing more to repel insects . . . but as I said the DEET evaporates which means it is disappearing and as the concentration falls below 25% so does the protection from insects. Using more concentrated DEET keeps the air around your skin saturated longer without having to reapply more repellent. However, you can also just reapply more of the lower concentration repellent.