Any finish which can stop grain-raising will also prevent weathering somewhat. The weathering when it occurs will be spotty and uneven until the application has worn off, which will also mean the protection is gone.
Perhaps paint/stain them gray, or let them go natural.
Any really rough spots can be fixed with a small sanding sponge...
Casey

I have an idea, but feel free to shoot it down. I'm looking into aging some pine, myself.

I've read that if you spray cedar with baking soda and water, it'll gray it quickly. Perhaps you could do that, then put your sealer on it?