Spring Cleaning, car care edition

Every Spring the sun comes out and I notice how filthy my car is.  This year it was especially bad because I'd damaged the poor thing.  So in addition to the usual wax-and-vacuum, I decided to touch up the paint and headlights.  I bought two kits, both of which basically involved polishing the surfaces.  I know a lot about polishing from my days as a metallurgist, so read on.

Field Test 1:  "Clay Bar"
Body shops and detailers swear by clay bar, which seems to magically erase dullness and tar flecks off of the outsides of cars.  It's better than soap but it's not paint - it can't fix scratches that go down into your car's paint job.  In my case, I had a huge smear of beige paint on my front passenger corner (more on that later) which looked ridiculous and could be seen from a mile away, so I figured I'd try it.

I am pleased to report that Clay Bar really works.  The clay bar itself is like a sticky putty. It's full of a very fine abrasive. The kit I bought (Meguiar's) came with a spray bottle of lubricant (mostly water). You work the clay bar until it's pliable, spray the lube on the car, and rub the bar wherever you're trying to clean something off. It very slowly removes everything on top of the clearcoat, basically by a polishing action. The clay bar doesn't seem to get used up, you just stretch and fold it to expose a clean area.  
(How does it work, technically?  The clay conforms to the surface, which reduces the pressure on the smooth clearcoat you don't want to damage.  But anything sticking up, like a fleck of tar, gets pressed into the clay.  It is, in wear science terms, an asperity.  The higher pressure between the abrasives in the clay and the asperity causes the asperity to wear down faster - eventually leaving nothing but the clearcoat.)
For me, it worked flawlessly. I went from having a huge ugly damage spot to like-new. It took about an hour. Unfortunately the temperature was in the mid-30s, and you have to work with your bare hands, so I had to go inside and run hot water over my hands a couple times to keep from getting frostbite. But it was totally worth it.
(How did I get that smear of beige on my car? I ran into my own garage. I had just been fitted for contact lenses for the first time in 20 years, and they wrecked my depth perception off-center. I just grazed the wood, so the body panels didn't get pushed in enough to be dented. There were just a few fine scratches at the high points of the panels where the clearcoat was damaged.)
Verdict:  awesome.  When it gets warmer, I'm going to go around my whole car with it.

Field Test 2:  Headlight lens restorer
I didn't know there was such a thing, but I'd noticed my 2004 car's headlights were pretty cloudy.  Like a fish that's been laying on the ice too long.  So when I saw a kit made by Turtle Wax next to the clay bar, I picked it up.  It was not an unalloyed success. 

The kit is a five-step process.  The instructions say to try Step 1, a fine polishing paste, and if it works for you, stop there.  When I rubbed on the paste using an old T-shirt, I could see a lot of brownish junk accumulating on the white cloth.  After one or two applications, the shirt stayed clean - and my headlights looked a lot clearer.  The general haze was gone, though there was still a blotchy mottled look on the part where the lights shine through.  The other four steps are basically sandpaper - you start with coarse grit and work your way down through the finer sandpapers.
(How do you polish a rough surface?  This process is familiar to any metallurgist; it's used to examine the internal structure of processed metals in a microscope.  You start with coarse grit and with each successive grit you rub at a right angle to the last, so you can tell when the finer scratches have removed all the deeper ones.  In metallurgy, the finest sandpaper is followed by slurries of even finer grit particles suspended in water to produce a mirror finish.)
This works great, except, of course, when everything is covered with a milky haze of ground-off plastic debris.  I can't tell you how many hours I spent in grad school polishing bits of metal, and I still screwed this up.  For the layman, there's a strong chance that it would actually make the headlights worse.  Hell, I didn't even manage to sand off the blotches I was trying to get rid of.

I was so alarmed by the way the first headlight looked after I'd gone through all the sandpapers that I only used the paste on the second headlight.  The final step is a cloth saturated with some kind of plastic-coating liquid.  You wipe this cloth on the headlights and it fills in the fine scratches and seals it with a smooth surface.  This was a huge improvement; it made the scratched headlight look clear again. 

Verdict:  Do the headlights look better?  Yes.  Good enough for me?  Yes.  Perfect?  No.  Would I do it again?  Probably not.