[Brent Deines]

Some customers prefer to use plate glass resin for filling pits. It is a higher viscosity and harder, which may produce a higher quality finish depending on your polishing procedure. I can't really tell any difference, but some technicians swear by it, and it is a bit less expensive than our pit resin.

If you check the refractive index of either of these products you will find that they are almost identical to glass, so other than the hardness I don't know what else could be done to make it more clear.

It may be that the company you are referring to is using a product similar to our plate glass resin. That's just a guess though, as I haven't seen anything that leaves a completely invisible pit, and we do test everything we can get our hands on.