On Mon, 17 Jul 2017 14:17:33 -0400, Frank wrote:

There is always grit to worry about on the road.

I've ridden a motorcycle in my youth for tens of thousands of miles, so,
I'm aware of what happens when you drive behind almost any dump truck.

I also own a sand blasting kit, so, I'm aware of the power of sand
blasting.

Still, my gut feeling (but no science yet) tells me that these lenses are
yellowed from sunlight alone, and not from being sandblasted. I suspect if
I left a car in the sun and never drove it, that the lenses would still
yellow.

I'm not saying that sand blasting can't happen - I'm just intimating
(without hard science) that it's not happening. I could be wrong. (That's
why I have my huckleberry friends here to advise me.)

I know the formula used for scratch resistant plastic but cannot divulge
it. It would basically be applied like a wax. That's why it may be in
a lens refinish kit but I do not know for sure. These finishes are also
used on plastic eyeglass lens which easily scratch. Composition I had
seen was for acrylic sheets.

The BlueMagic MSDS says "acrylic copolymer" as an tiny percent ingredient.
http://bluemagicusa.com/index.php/bl...lens_restorer/
It also lists Neodol, Argilla, and Pluronic F, none of which sound familiar
to me so I'll need to look them up but they're in tiny percentages also.

Both acrylic and polycarbonate have good outdoor resistance but both are
much softer than glass and easily scratched and dulled. Years ago I had
a watch with plastic lens that I occasionally restored clarity with
tooth paste. Toothpaste contains particulate that will not scratch teeth
but is a mistake to use on plastic dentures as it could wear them. I
have an old dental bridge that once had a plastic coating. Now years
later after years of brushing it is gone and all metal.

I think everything "gritty" will be harder than the plastic that is used on
those headlight lenses. I'm thinking the Bentonite Clay that a well driller
gave me is the right size (it's a powder) but if I can get a hold of
aluminum oxide powder, that would be a good second bet.

The toothpaste grit is often silicon dioxide (sand) but they used those
blue plastic beads for a while (I hope they stopped that by now).

The trick, of course, is to get the right grit.
The good news is that the worst thing that happens is we ruin an already
ruined headlight - so luckily experimentation is not risky.