I have Wuuthrad from the Companions quest line, realised it's base damage is not as high as my Skyforge Steel Battleaxe (Superior) and went to upgrade it (Smithing skill is 65).

Unfortunately, Wuuthrad doesn't even show up on the list of upgradeable items . . . There are other items that show up but are greyed out due to insufficient skill.

I guess this means I cannot upgrade it at all, but does anybody know this for sure?

I duel wield two flawless katanas and their damage is 40 each I believe. but I don't think you can upgrade it.

Isn't that enchanted though?
Then you need a perk

I think you need the Arcane Blacksmith perk.

Yeah I took the Arcane Blacksmith perk once I got Wuuthrad but no luck!

Oh well, it's still a cool axe, might need to relegate it to ornament status soon though.

You can actually enchant Wuuthrad.

I'm waiting for a mod to make all of the unique items that you can't upgrade or can't upgrade all the way upgradeable.

Sounds like that's the way to go, then. That mod you are waiting for would be sweet, a pity I'll probably never get to see it (playing on Xbox)

I'm hoping it gets bundled with the mod to make all of the Artifacts in the game no longer svck.

can i upgrade weapons past epic?

Actually it's normal for some artefacts, daedric or not.

Yes, it goes up to legendary. If you use enchantments and potions to improve smithing, you can get better legendary equipment, but it is still called legendary.