Very High Grit Sandpaper Application Question

[jpanhalt]

320 for headlights?  I would start at 1500.  If that doesn't work get coarser until it does.  You will need to finish higher than 1500 unless you have power polishing.  There's a difference between shine and scratch free optics.  For example, electropolishing can give a high shine on a relatively rough surface.  When you get to the end, a power buffer gently applied will be appreciated.

[Psi]

I'm planning on polishing my headlights, but I won't be buying any kit from HF or anywhere else, as I already have all the materials I need: 320/400/600/1500 wet-or-dry silicon carbide paper, rubbing compound and polishing compound. Oh, and nice soft clean rags.

as jpanhalt says, 320 is too course you will add scratches only to then have to remove them.
Perhaps if your headlights are deeply scratched you might need 600 or 800. but not 320.

Do a test on a deep scratch to figure out how fine you can start with while still removing the scratch

[coppercone2]

the problem is sealant they always go bad after a while

supposedly it has some anti oxidant coating. I tried the kits with the liquid but it still goes hazy after a while. if your lights are at this point, it turns into a maintence procedure. Not a bad way to hold off another year or two, but their degraded

[Psi]

the problem is sealant they always go bad after a while

supposedly it has some anti oxidant coating. I tried the kits with the liquid but it still goes hazy after a while. if your lights are at this point, it turns into a maintence procedure. Not a bad way to hold off another year or two, but their degraded

It's almost like they were better 40 years ago when they were actually made of glass.

[jpanhalt]

Almost?  Glass was better, but maybe a few pennies more to have molded in today's shapes.   My 1965 VW had glass lights that you could replace without reprogramming anything.  Despite all its so called conveniences, I do not like my 2024 Mazda CX-30.  There are a few things that stay set between starts, but some others are complete nagware.  Cruise is a real annoyance.

[Analog Kid]

320 for headlights?  I would start at 1500.  If that doesn't work get coarser until it does.  You will need to finish higher than 1500 unless you have power polishing.  There's a difference between shine and scratch free optics.  For example, electropolishing can give a high shine on a relatively rough surface.  When you get to the end, a power buffer gently applied will be appreciated.

You're right, of course.
I just put 320 as a starting point because it's the lowest grit on my scale; I would probably start at 600 for my headlights. I have patience, so don't mind spending more time on each grit to remove previous scratches.

I'll look for grits above 1500 at my local home improvement store (not Home Despot!). And the polishing compounds start off from there (coarser "rubbing compound" (reddish brown), "polishing compound" (white)).

[Monkeh]

3M do a very simple kit which is 500, 800, and 3000 grit, plus a polishing pad and compound. Just add drill, takes about ten minutes a light. Easier than digging up all the loose bits..

[jpanhalt]

Your home improvement store probably won't have high grits.  Any auto supply that sells finishing materials will or should.  Aside from industry, their main use is "color sanding" auto finishes.

[CatalinaWOW]

If you have the patience to start at 1500 either good on you or you are very picky about your headlights.  The problem isn't scratches, it is a thick layer of degraded material.  Can't say. Whether it is UV exposure, oxidation, outgassing or whatever.  On my twenty year old vehicle it took quite a while with a power tool and 400 grit to cut through the yellowed crud.  Then you can start polishing which actually goes pretty quickly. 

I agree that it becomes a maintenance chore, but between the lack of availability for older lenses and their high cost it is well worth it.

[mzzj]

I remember using was 6000 grid

Wow. I thought 800 or 1000 was smooth.

if you need really fine grit the magic word is "lapping film" instead of "sandpaper"

3M lapping films are available to 0.05 microns that translates to about 500 000 grit. This would be wayyy beyond what is considered mirror polished.

[shapirus]

3M lapping films are available to 0.05 microns that translates to about 500 000 grit. This would be wayyy beyond what is considered mirror polished.

Wow. I wonder what the application would be. UV optics?

[jpanhalt]

As mentioned, I used them to make electrodes from glassy carbon for an HPLC detector.  They worked very well.  I believe the smoothness may have decreased absorption in the detector.  The main advantage by far was that glassy carbon allowed virtually any solvent to be used as compared to graphite paste that was used before.  I did not go to 0.05 micron as I recall, but the grit I used was finer than DuPont color sand paper.

[Kleinstein]

Very fine sanding / polishing is wanted for metalograpic smaples. This can be optical, AFM and SEM. Usually the fine part it done with pastes, but a paper can be faster.

[mikerj]

320 for headlights?  I would start at 1500.  If that doesn't work get coarser until it does.  You will need to finish higher than 1500 unless you have power polishing.  There's a difference between shine and scratch free optics.  For example, electropolishing can give a high shine on a relatively rough surface.  When you get to the end, a power buffer gently applied will be appreciated.

It depends on if you have to remove the remnants of the original anti-scratch coating or not.  It can craze badly and get patchy on old headlamps, and you'll be there for days trying to remove it with fine grit.  The finish doesn't last long if you have to do this though, even with a UV protector, as polycarbonate is so soft.

[jpanhalt]

That's precisely why you start with fine and get coarser, at least for someone who doesn't do the job everyday.  Once it's hit with coarse grit, you are committed.

As for crazing, my only experience is with acrylic windshields.  In that instance, it was due to a combination of stress, wrong sealant (mineral oil-based is bad), cleaning with organic solvent based cleaners, and exposure to sunlight.  It was not not a coating and was not removed appreciably by polishing.  A picture at the outset would have helped.

[Analog Kid]

The finish doesn't last long if you have to do this though, even with a UV protector, as polycarbonate is so soft.

I wonder if this wouldn't indicate hitting the headlight plastic with some kind of clear coat (like lacquer) after polishing.
Would have to be done very carefully, of course, to prevent runs, sags, orange peel, crazing, etc. Not to mention masking off surrounding stuff.

[jpanhalt]

I know you said "like," but I would never use solvent lacquer on plastic.  A water-based clear coating like epoxy or preferrably polyurethane would be safer (e.g., Minwax polyacrylic).  The problem is crazing or fogging.  It might also be an odd use for acrylic floor wax.  That wouldn't be very durable, but it is easily removed and replaced.

[coppercone2]

glass is slightly less translucent then acrylic, so you have attenuation. Maybe it was a tiny benefit when the lights were still low, but the LED are so bright now, who gives a crap, they say its blinding drivers.

What would be nice is sapphire crystal like a rolex. Maybe this is the improvement we will see with cheap energy that makes it cheap to make high grade ceramics and glasses in cars of the year 2500. But I think they used to be able to say that the light is measurably brighter when acrylic housings were new, for marketing. That feature turned into a hindrance now.

The only issue might be thermal management, its possible if glass does attenuate more, it might get too hot, its possible its a issue with brighter lights. But you would think if this is true, then the scratched up fogged lights would be melting

https://longislandwatch.com/sapphire-crystal-mods/

I guess a headlight would be like $2000

https://www.bmw.com/en/magazine/design/swarovski-crystal-headlights-in-BMW-i7.html





ah but it seems that most lights are made from polycarbonate, which is less translucent then glass, not acrylic.

[CatalinaWOW]

It is the stylists and industrial engineers, not the opticians and physicists who are setting the material specs for headlights.  Well there is some tiny contribution from the aerodynamicists. 

Somehow the car buying public has been convinced that the shape and visual appearance of headlights is really important.  So just about every model has a unique shape.  Hence relatively short production runs (tens to hundreds of thousands, not the millions to tens of millions for the old sealed beam headlights).  And complex shapes which are harder to mold in glass.  Hence polycarbonate and perhaps other plastic materials.

Super bright LED light sources don't really help, because the problem with older headlights isn't optical transmission, but scattering.  While the transmission might have gone down by 10-20%, the light delivered 50-100 meters down the roadway has gone down by a factor, perhaps in the neighborhood of ten in a severe case.  The rest of the light is off illuminating the clouds and the fields and buildings off to the side of the road.

[bostonman]

Super bright LED light sources don't really help

Not sure if this is country wide, but in MA, seems the new thing is driving with the high beams on all the time at night (they don't even turn them off when you flick your high beams at them), so doesn't matter about super bright LEDs or hazed headlight lens.

Then one car has super bright white headlights, another has blue, etc...  Seems the standards for headlights have changed. Some SUVs have headlights so bright you'd swear they have their high beams on; maybe they are aftermarket but have heard from owners their headlights are stock but people flick their high beams at them thinking they have their high breams on.

The other new way of driving, at least in MA: sit at a green light (most likely because the driver is staring at their phone or 52" control monitor screen on their dashboard), but go through a red light that has been clearly red for a solid three-seconds.

[SteveThackery]

Another use I have for lapping films is for tuning fountain pen nibs. I use 5, 3, 1 and 0.3 micron films. Obviously the initial shaping is done with 5 micron, then progressively finer to polish the nib until it gives the desired amount of "tooth" (resistance as it moves over the paper). For a toothy nib I usually stop after using the 5 and 3 micron films. Applying the 1 micron film makes the nib noticeably smoother, and the 0.3 micron film lets the nib feel like it's almost floating across the paper.

Although a floaty nib would seem desirable, many people (including me) find that their handwriting is neater when the nib has some drag on the paper.

PS: toothiness is not the same as scratchiness. A scratchy nib tears the paper fibres and drags them along with the nib, causing blotches and other unwanted marks.

[jpanhalt]

Do you  polish differently for lefties and righties to correct for the angle and strokes?

[SteveThackery]

Do you  polish differently for lefties and righties to correct for the angle and strokes?

The experts - "nibmeisters" - do, yes. I don't because I'm just a hobbyist doing my own pens.

[jpanhalt]

Esterbrook didn't (circa 1950's).  Was it not an expert?  I'm glad you know the difference.  I would return to ink in an instant if I could find a decent pen at a reasonable price for a leftie.  Otherwise, I am stuck with Pilot ballpoints.