Houston Husker - The pros already said it, once it starts to yellow, there's no way to reverse it.
I'd like to know if you used this product:
http://www.rustoleum.com/CBGProduct.asp?pid=13
I think Rustoleum only makes one clear. What epoxy did you put it on top of? Rustoleum brand? Water or Solvent based? I'm asking because I put down Rustoleum Solvent epoxy and coated with Rustoleum Solvent Clear. My Garage Door faces west and is only open when cars are entering/leaving. I also have a 18x24 window facing north with no shade.
I installed approx 18 months ago and have seen no yellowing whatsoever.
So I'm wondering if your clearcoat is yellowing or if the underlaying epoxy is yellowing.