Hey ya'll

I have about 20 Oak beams varying from 6x4 to 8x8 that are rough cut and very gray from being out in the weather for a couple of years.

I'm getting ready to build and I want to clean them up first (that is, I would like a nice light color with the wood grain showing). I remember hearing someone talking about using some type of chemical to refresh the wood and I have also heard about using a pressure washer, but I'm skeptical that anything could take off the gray and remove the sawmill marks as well as a power planer. However, I figured I should ask before I pull the trigger.

thank you! (I did search the forum first)

That's the route I'd take personally
or a hand plane if I had the time.

Planing or heavy sanding is the only way to brighten them up and certainly the only way to get rid of saw marks.

Do you need them smooth or is it more of a color issue?

Have them resawn on a band mill. It could also be a chance to true the timber.

They don't need to be super smooth. In fact, a little rustic is fine. However, I want them to be light colored and to show the grain. Right now they are simply dull gray and fuzzy. I tried sanding, but the gray goes pretty deep and sanding didn't remove the saw marks (and the gray in the dips between the saw marks). A band mill creates lots of logistical and $$ problems for me. It seems like a hand power planer is the way to go. A buddy has one he insists on loaning to me with a "jig" for sharpening the blades.
Once I plane the beams, I will cover them with cotton/poly tarps until we start building in August. I'm assuming the tarps will keep them from re-graying. Although, I better make sure there is good ventilation. Thanks!

No matter what you do, the timbers will 'grey' to a certain degree. Wood cannot keep its new color forever. The only way to stall it is to put some kind of UV-protection over it, and personally I'd rather it be grey. Granted, this will not be the same as leaving them in full sun, but it still is something to note. UV, water, and a slew of other issues will fade the color and otherwise change it.

For oak, they'll dull down quite a bit after a few years, they'll never stay as bright as newly exposed wood. But again, I still agree planing down to expose fresh wood is better than leaving them, for a number of reasons.