I have had DEET melt plastic ball point pens (white plastic body), melt rubber on a rubber rain suit, ruin the finish on the plastic temples on eyeglasses, melt the plastic face of several cheap watches, and weld a fly line. If I am going to use DEET, I now try to wear an outer layer of clothing that I spray instead of spraying bare skin. I definitely don't apply it to the palms of my hands or bottom of my fingers.
I like the roll-on applicator bottles when I can find them, followed by spray cans, and then ones I have to apply by hand last. I wear long sleeve shirts that have been sprayed, a bandana around the neck, spray the underside of the bill of a ball cap or hat, etc.