Guys, I'm a detailer and I would like to shed some light on this from my point of view. I see al kinds of wacky ways to restore headlights and I hope I can clarify some things here becaue it's really pretty basic and doesn't take a lot of time, effort or supplies. I'm going to share with you what I do, from a pro's end of it and hopefully that will help.
First let me say, becasue I have not mentioned specifics about my detailing business since I've joined here, that I have been detailing cars, sailboats, yachts (up to 60 ft), semi trucks, etc, etc. for 22 years out of love for the work. It all started by accident becasue I am a clean fanatic and HATE anything messy or dirty. I got into detailing before I was driving! I worked at a local detail place and got tired of the mentality that they wanted to hurry that car out to get the next one in, just to show numbers at the end of the day. I don't work like that and when I do something I want to do it right so I went and did my own thing and I am blessed to have wonderful and extremely loyal clients - and as of now almost all my new work is via referrals and I do very little advertising.
Through the years, I have worked on some very cool cars and for the last 8 years I've been the official detailer for the San Francisco 49ers - players, coaches, staff, etc and I've detailed super-high end vehicles for people who are tops worldwide in real estate. I never get tired of the work and if anything it helps me maintain the same weight I was in 8th grade (I'm 41 now) and that's a good thing because I eat like crazy but maintain 6'2" and 165lbs year 'round LOL!
My main thing is this: DO IT RIGHT. Use the right product and use it properly. If I don't know exactly what I'm doing, I stop and find out what is the best and proper way to deal with the situation. With the internet on our phones these days, there is no excuse.
I absolutely love the business and could go on and on but I'm going to get to the subject at hand. I will probably repeat some of what's already been said but I'm going to start from square one.
The yellowing is the top layer (and from what I understand the UV coating) on the polycarbonate lenses oxidizing. Basically, the sun's UV rays has killed that layer and it is "dead". Washing your vehicle with anything other than car wash soap can also aid in the yellowing of the lenses as well - so again - use the proper product per the situation.
In order for the lenses to look like new again, that dead layer of material needs to be removed and then restored properly.
Here's what I do: I mask off all surrounding paint with blue painter's tape. NO DUCT TAPE guys and no electrical tape, especially in hot weather. Use painter's tape and cover the paint about 2 inches back from the lenses.
The lenses need to be wet sanded to remove the dead/yellow material. The grit you start with depends on the severity of the oxidation. Typically when sanding I'd recommend using 1000 grit followed by 1500grit and I finish with 2000 grit. Pesonally, I only use 3M sand paper - it's personal preference and I believe they make a quality product. Keep a spray bottle of water handy and be sure to keep the sand paper wet as you are sanding - don't let the sand paper dry. If your headlights are severly oxidized, you may need to use a more coarse grit below 1000 but I find that most of the time 1000 is a good starting point and I always start with the least aggressive method first - on some occassions that means I have been able to clear mildly oxidized lenses starting at the compound stage alone but that is because I use a high speed polisher which makes extremely quick work of it.
Here's where I may loose some of you unless you have experience using a high speed polisher. You can do it by hand but it's going to take longer and you probably won't get as good of results as using a high speed. If you hand sand, I'd recommend finishing with a finer grit than 2000. After sanding, using a high speed polisher, I use 3M compound on a white 3M foam compounding pad to remove the sanding scratches.
After compounding, before moving on to the polishing stage, all splatter and any trace of the compound MUST be removed by wiping it with a microfiber towel - I use a slighly damp towel. Compound contains abrasives (I know some of the new stuff doesn't feel gritty but it is still intended to be agressive and you don't want to carry any trace of that into the next/polishing step) and you want to remove any and all traces of it before moving to the next step.
Next, to remove the compounding scratches, again, using my Makita high speed, I use 3M Foam Pad Polishing Glaze (I believe it's been discontinued but I bought a large amount a while back!) or 3M Machine Polish and work that over the lense and when you are done you should have a perfectly restored lense.
The lense does need protection at this point and I apply two coats of a polymer sealant followed by two coats of a good wax to the lenses. One of the sealants I use is from Pro Products, called Polymer II (you can read about it at prowax.com). For wax, I like Satin Creme Wax, it's a favorite of mine, again from Pro Products. Use foam applicators to apply the products and you will get good, efficient results with minimal product. What I do for a foam applicator is when my 6.5" pads for my Porter Cable wear out, I remove the velcro from the back which makes a giant applicator or you could cut it into smaller pieces if you like. If you have a Porter Cable DA, by all means use that - that's what I do because it produces far superior results vs hand application and in far less time.
As some have mentioned, if you wax your vehicle regularly, waxing your headlights is a huge step in preventing the yellowing lenses in the first place and I highly reccommend doing that - it literally only takes a couple extra seconds to do! I wax my Astro almost bi weekly just becasue I love doing it, not that it needs it and I wax all windows, headlights, brakelights, side view mirrors, antenna, wheels, badging/nameplates, grill, windshield wiper arms - everything that is non-porous gets waxed.
You can also be careful how you park. If you are always parked facing the sun, your headlights are going to take a beating. Look around - there are cars with one yellowish and one crystal clean lense - it's because the sun hits the yellowed side of the car every day and takes its toll over time.
As far as products to use, I would offer a word of caution regarding using a chemical that is not intended for a particular situation. You simply don't know the side effects or long term effects that it will have AND what it will do if it touches the paint on your car. My thoughts are unless I've done research to confirm, I dont' know if it's safe to use a particular product ouside of what it's advertised and designed for. Be careful!! I have heard toothpaste and peanut butter work but think about this - If the dead layer of material needs to be sanded off and physically removed to properly remove it, how is peanut butter going to do that? Some of the "fixes" for restoring lenses are just temporary and the yellowing will return soon and you'll have to do it all over again. Peanut butter may appear to work becasue the oils in it may wet the dead layer of material - just like products that claim to "remove swirls" don't becasue they just have a bunch of silicone in them and silicone hides but does not remove scratches - that's a whole article in it'self but trust me on that one - don't use polishes with silicone in them if you want to remove scratches.
Also, once you have performed the headlight restoration process, the lenses will need to be waxed on at least a somewhat regular basis because remember, that original UV coating, which was inevitably much much thicker than waxes or sealants is now gone and if not waxed on a regular basis, the lenses will eventually yellow again.
I hope this helps and I have tried to be brief without turning it into a book but there are so many specifics I wanted to add! I sure hope leaving those little things out didn't confuse anyone. I enjoy anything that has to do with detailing and maintaining a vehicle cosmetically. If you have any questions, please ask. I am busy but I try to check in here as much as I can but I do have a storm of work coming in over the next couple weeks
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I should note that the products I mention here are actual products that I use and I have been using for nearly the entire time I've been detailing. I am not associated with them in any way other than using their products on my client's vehicles. 3M can be pricey but it is worth the price. Pro Products has been around for a very long time and has some absulutely awesome products. Don't go cheap on products - you get what you pay for. I have tried all kinds of products and the better ones may cost more but for me it's worth the efficiency and "works as advertised" that I get from purchasing good products.
I can restore a pair of headlights/lenses in about 30 - 45 minutes, depending on the severity of the oxidation. If you do it all by hand, it should take maybe an hour, hour and a half but you can do it, it's not really that difficult. If you have another vehicle that has this problem, be efficient with your time and do them both at the same time. Now get out there and get to it! LOL