Apply UV Protector Over Stain?

Recently, I power-washed, cleaned, brightened/neutralized and sanded my pressure-treated ten-year-old deck. (It took over a month to do all of this back-breaking, knee-killing work.) Then I applied Sikkens Cetol SRD in the Chestnut Brown color.

The deck turned-out beautifully, and I want to prevent it from turning gray for as long as possible, so I began researching whether there was some sort of UV-protective coating/sealer that I could apply OVER the existing stain. I read about Defy Extreme Stain, with its nano-technology and zinc oxide. The problem is that this is only for applying on freshly stripped wood. The truth is that my deck is still really porous (water absorbs in a few seconds) so maybe I could get away with applying the Defy over the existing stain? On the other hand, maybe there is some other product out there that has zinc oxide in it, AND that is meant to go over existing stain.

I wish that I would have found your site before I started the project, but now that I'm here, maybe someone can help me figure-out what to do. I live in Minnesota, and I'm afraid my deck will be gray by springtime.

Thank-you!!