Why headlight polishing
Many headlights are made with a transparent plastic (polycarbonate) cover. This cover protects the sensitive light bulb inside and also prevents the reflector from becoming dirty. In the last years, these have also become a styling component, creating shapes that have a cosmetic effect on the overall appearance of a vehicle. These headlights can be found on virtually all vehicles. This transparent casing can turn dull over time, apart from the cosmetic and visual difference, it also has an effect on the light that shines through the casing. The dull layer will reflect light in several directions, possibly blinding other traffic members. This means you can fail your MOT or similar. It is also a cosmetic thing, where the dull and yellow-ish headlights make the car look tattered and worn.
What are you doing when polishing headlight
The outer most layer of the plastic casing has suffered from UV damage. It has started to oxidize, and this oxidation needs to be removed to reach the “healthy” material. Only polishing will most likely just flatten the oxidized layer, but not remove it. This will speed up the return of oxidized surface. Removing the oxidation might “cure” the problem for a few years, just polishing out the contaminated material could make it return within a year.
Sanding is a very common practice to remove the faded material, but sanding to rough can cause surface imperfections called sanding marks, or the smaller “pigtails”. The marks are grooves in the sanded-down surface that are deeper then the rest. When you start polishing, you would need to polish as much material as the depth of that groove. In some cases this means polishing away much more material then needed, which also costs more time and more product than is needed.
After sanding to the correct depth, the surface is then polished to flatten out the surface. This perfectly smooth surface will allow the light to pass through it freely without being bend or reflected in any incorrect direction. It will also make the headlight look like new again.
How to polish headlights
Polishing headlights can require a slightly different approach each time. The difference in material, level of oxidation and even the shape or size of the headlights can require a different approach. This means that the steps for polish headlights can be divided into 3 basic steps.
The wetsanding stage
In this stage, the goal is to remove as much of the oxidation as possible without removing more of the material then is needed. The sanding is most commonly done by wetsanding. The general advice is to use automotive wetsanding paper, because these have a more even sized sanding-grain and minimize the risk of sandmarks and pigtails. In the photo to the right, you see a sanded surface. This surface looks cloudy with an uneven finish. This is evidence that you need to do more sanding. The cloudy look with patches of different dullness are due to the amount of oxidized material that is being sanded. In fact, the dull patches are the oxidized areas, because these are softer and turn dull quicker when being sanded.
It would be impossible to say what grid you need to start with, because it depends on the hardness of the material, the level of oxidation and many more factors. In general I would be prepared to start with 1000, then follow up with 2000 and finish with 4000. To make the polishing step easier, some detailer prefer to follow this up with 6000. In some cases you’ll notice how a certain manufacturer doesn’t provide a product with grade 2000. In that case you take the grade that is closest by. For example: 1500 or 2500.
Wetsanding should be done with plenty of water. A spraybottle with clean water is a commonly used setup. You’ll also want to clean the sandpaper regularly to avoid getting pigtails or sandingmarks. Never start with the most rough grade! Always start with the finest grade and work your way up till you’re happy with the result. Then work your way down again to the finish grade.
The polishing stage
After the contaminated material is removed, and the finish is smooth and even (not cloudy), the headlights can be polished. The polish used at this stage can also slightly differ, but in general you want to start with a rough combination. Some detailers opt for a cutting foam pad with a polishing
or cutting compound, others prefer a microfiber or wool cutting pad with a polishing or cutting compound. The cutting at this stage is needed to remove any sanding marks, polish away any pigtails and to flatten out the surface you have created with sanding.
If the headlights where not dull enough to require sanding, you might want to start polishing with the least agressieve combination. Work your way up till you’re happy with the result. Then work your way down again towards the finish combination for a smooth and flawless finish.
Because some manufacturers make polishing liquid specifically for plastic, some detailers choose to finish with plastic-polish on a medium setting on a finishing pad. It should give the perfect finish on plastic material.
The protecting stage
Headlights turning dull is very difficult to prevent. Many different factors play a role. The amount of time the headlights are exposed to direct sunlight, the amount of surface damage they suffer (for example from stone chips), the place in the world (certain places get more hut burning sun then others) and certain chemicals the headlights may be exposed to. Protecting the headlights can help to prevent them from turning dull again. This protection either forms a barrier between the surface of the headlights and the air (like a clear coat), or it can filter out harmfull UV-rays from the sun, decreasing the level of harmfull rays that reach the plastic surface. Both method are an option, depending on the situation.
The most common forms of protection are: clear coat, protection film (like a transparent foil) or ceramic coating. Some also use wax or sealants, but it is not proven that these actually have any contribution on preventing headlights from turning dull again.
Things to look out for
When polishing headlights, there are certain things the detailer might want to keep an eye on.
- Keep on eye on the temperature. Plastic doesn’t dissipate heat very good, meaning it gets hot. When it gets to hot the polishing pad can get sticky, you risk literally burning the surface and the polish will become much less effective. If its to hot to hold in your hand, it is to hot to polish.
- Headlights are strong, but not unbreakable. They can break if you use to much pressure.
- Tape off the area well, you don’t want to hit any painted panels with sandpaper.
- Remove the light bulb and the driver (in case of Xenon lights). These don’t react well to intense vibration from a polisher.
- Removing the headlights from the car will make it much easier to work with.
- Some headlights are made from glass, these shouldn’t turn dull but also shouldn’t be polished.
- Using proper products, machines and procedures will make the job a lot easier and give much better results.
- Dull headlights can not only be a failure on your MOT due to danger of blinding others, it can also make car look tatty and worn. Polishing these will improve the overall appearance.
- Although toothpaste does have some effect on the dullness, it is not strong enough to fully remove the contaminated material. It will improve the overal looks, but it is not a dedicated polish. If the proper products and tools are available, using these will give much better result. However, if you are in need of a quick-fix, toothpaste can make a slight difference. Just rub it in really well and rinse/wipe off.
- There is also a trick where the steam of boiling vinegar is blown over the surface of headlights. Unfortunately, this is just a very temprary trick where the surface of the headlight absorbs moisture and shows much less off its dull surface. As soon as the vinger is washed off, the dullness will be back.
- It is impossible to prevent any form of oxidation completely. You can slow it down for many years, but 100% prevention is not possible. In normal situations, you would only need to polish the headlights once every 2 to 5 years. If the dullness returns quicker, it wasn’t protected properly or the contaminated material was not properly removed.
- The fact that you might not see any more dullness after polishing, will not always mean that the oxidized material has been removed from the plastic.
- If you decide to spray a clear coat over the headlights yourself, it is advices to use 2 component paint to provide the best level of protection.
- Automotive sandpaper gives a much more fine result, that require a lot less refining then wetsanding paper from the bodyshop.
- Only spraying clear coat over the dull headlight can make it look clear and transparent, but the dullness will return shortly, even though it is covered in clear paint. The clear coat can even make it less easy to properly remove and repair.
- There is also a trick where WD40 is used to add transparency to the headlight. This might seem to work in the beginning but will loose its effect as soon as the WD40 is washed off.