Plastic headlights ideally need buffing back to clear with a rotary polisher - I've been doing my E60 BMW lights recently. Even with a rotary machine its a ballache. What doesn't help as it appears they've got a cloudy layer on the inside, so even though one of them has polished quite well on the outside and I've removed most of the pitting... it still looks a bit shit :(
I'd be inclined to paint the AC pipes rather than cover them with something, as inevitably water etc will get between the covering (be it rubber pipe or tape or whatever) and the pipe and sit there corroding away. That's what happens to the discharge line (big pipe) on a P38 along the firewall foam. It sweats in operation, the condensate is absorbed by the foam, and it then sits there corroding out the back of the pipe resulting in pin holes so small it takes a nitrogen test+soapy water to find them. And of course as Richard says - the condenser that 100% of the time rots out the top corner thanks to the block of foam LR put on it. Gets wet from rain etc through the grille, rots out condenser.