Headlight polishing update
 

In August of 2019, I decided to carry out some long term experiments to solve the issues with my MkII's opaque headlight lenses. https://the75andztclub.co.uk/forum/s...highlight=lens

To test my theory I scrounged a gash Mk I lens from a local member. Idea was to try ordinary clear coat lacquer on on of the lenses on my Mk II and on the donated MK I lens, the latter being left out in the full sun and weather for a test period to see what developed. My Mk II spends much of its time in my garage, so hardly a fair test of deterioration.

I began by thoroughly abrading both headlights down to bare polycarbonate, with course wet and dry, until I was certain all the original coating was gone, then polished them back up, with finer and finer grades of wet and dry, finishing off with a metal polish, before washing thoroughly.

That, I followed with several coats of my clear coat aerosol. My finish proved a little disappointing, in that marks in the form of what looked like cracks appeared in the coating on both lenses.

I managed to polish the worst of the lines out, before finishing with metal polish. Too the good - neither lens has shown any deterioration at all in the 6 months since I did them.

When I have simply polished them up without any coating, the have deteriorated almost before my eyes in just weeks - so I'm that a successful experiment.

Polycarbonate is very unstable to sunlight unless it's coated with a UV protectant. Spraying the repolished lenses with a UV stabiliser is essential or all your time and elbow grease will be wasted. I wonder how many of us actually do that?


TC

It can also depend on how the lenses are moulded. A small decrease in mould temperature would upset the length of the polymer chains making them more prone to degradation than lenses moulded at the correct temperatures.

Would the effect driving through rain, dust smoke even air cause more immediate issues with abrasion or breakdown of the clear coat?

Perhaps also the uv reflecting of the surface of the polycarbonate to the inside of the clear coat cause issue?

I understand the experiment is just a simple one, but could the questions above be even relevant or am I thinking too much into it.

I have to say I find your experiments both engaging and fascinating. I and I am sure others appreciate the time and the effort you go to for them. Thank you.

Sent from my SM-A600FN using Tapatalk

Virtually all the lens polishing kits I have seen come with a protective clear coat that you are suppose to apply after polishing and then renew every so often.

Like you I have a MK2 and I used a standard headlight polishing kit to fix it. Not a showroom finish but I got allot of improvement. The trick is to be methodical and make sure you sand thoroughly with each pad.

It is exposure to UV which clouds the plastic, so even if doesn't have a protective coat as new, it is sensible to apply one after polishing.

just as info, wifey Liz's lil Zr has plastic lens and being an 04 plater , its the same age and sits out in all the same weathers as the ZT's.But to date the lens are still pefect, not a spec of discolouring ;).

The 06 1.8T mk2 's lights are awful after a year or so since a polish session, the Mk 1 CDTi's are due for a clean as just one side has gone off since a polish a couple years ago.

And the Mk 1 260 lights , again an 04 are still very good and never been touched

So no rhyme or reason or nor particular model to blame


So must be the plastic type and the makers ! :D

C

It doesn't affect the polymer chain length, but it certainly impacts on the physical properties of the finished article.

Spotted this: Mold temperature has perhaps a less obvious but often more profound effect on final properties. In amorphous polymers such as ABS and polycarbonate, higher mold temperatures produce lower levels of molded-in stress and consequently better impact resistance, stress-crack resistance, and fatigue performance.

Of course, in the real world environment, plastics and their coatings are affected by abrasion from dust, etc. Frequent freeze/thaw conditions also pay their part in fatiguing the lenses.

The UV that degrades polycarbonate headlamp lenses comes from sunlight, which is absorbed by the sacrificial coating. Compounds in the coating react preferentially with UV radiation preventing it getting at the lens itself. The reactive compounds are permanently changed into non-absorbing chemicals. As their activity drops, the underlying polycarbonate will start to degrade. There's little if any UV generated by the headlamp itself, so lenses don't require an internal coating.

I recently repolished my headlamps for the second time after not giving them any protection the first time. They're still off the car (SORN) until the weather warms up a bit. The expensive UV protectant I intend to spray on needs somewhat milder ambient temperatures than we have at present.

Some polishing kits do come with a wipe-on type lens coating which I suspect is next to useless. They will give a very short lived UV mitigation, but you really do need a solid plastic film rich in UV absorbent to do a proper job. Even so, none of this is permanent unless you keep the car in perpetual darkness.

TC

The reason I was asking, particularly about the reflection of the UV, is some years ago I can recall reading about a lens that had various coatings on it. Not referring to any UV coming from the bulbs themselves. It was supposed to be a high end lens, and indeed it was, except for bright light. There was a lot of flare with this lens, and after heavy investigation, it was explained that one of the glass coatings caused an internal reflection between the coating and the glass it was coating (talking microns!). Am probably thinking too much into it lol.

I have had those kits with the 'UV' protectorant. I have no doubt that it washed off lol. The ones I have used reminded me of Autoglym's extra gloss protection sealant, in its application.

Perhaps Rover should have developed a pair of mechanical eyelids for use during the day.---Like these---:cool::cool::cool:

Yes, interfaces - the place where two layers touch - is a natural place for light reflection. Maybe a simple example, but the interface between a swimming pool's water and the air above produces a mirror at times and this causes internal reflections as seen by swimmers underwater. In the example you mention, the internal reflection is from the headlamp bulb's light traveling outwards and some being reflected backwards at the coating interface. This light will also get re-reflected outwards, but in spurious directions. It's that which creates flare/glare. In the case of a UV coating, the UV radiation is coming in from outside, so interfacial reflection will be back out again, so not affecting the polycarbonate further in. Interfacial reflections is why optical lenses for telescopes, cameras and such are given molecular-thin coatings of various materials to minimise image ghosting. It gives lenses a colourful caste when seen at a certain angle.


TC