Hxtal NYL-1 is the only epoxy that will not yellow over time that I
am awareof. Apparently, the iron molecules in epoxies causes the
yellowing over time, and the company that makes Hxtal takes
considerable pains so that the number of iron atoms are minimal.
There are a couple of independent studies that you can find to
support this, if you search it on the web.
Having said that, Hxtal dries slowly, and is quite expensive. I find
myself using Devcon or other low-cost epoxies as well for some of my
jobs, particularly if the epoxy is not visible or will not become
visible. But I am aware that they will yellow over time. For highest
quality work, or when the epoxy is visible, as with clear items, I
do use the Hxtal. Lately, I have been organizing my work so that I
do have the time to allow the better quality Hxtal epoxy to set. I
just feel better about using it.