Whetstone so you can sharpen your blade after forging the weapon so you can sharpen it, or even sharpening it before going into a cave as a buff or maybe to repair a weapon too. You can probably have different type of whetstones. Where some are better than others. Just an idea if you want to add an extra layer to the crafting, repairing, or buffing.
So different whetstones do different things? Like, one whetstone repairs the weapon and another gives a temporary damage buff?
I think the idea is different types would just be better (in both), as he said. I could see whetstones being used in the prosses of repairing blades, as after you add more metal onto the blade (say heating both the blade and bar and then hammering them) it would need to be re-sharpend.
Personally I think it would be better if the damage boost the blade got from sharpening went away based on durability used, so it’s best to get it shapened in between a lot of use instead of a timed buff.
Also giving ir the trade off of using up some durability while sharpening it (like in real life) so it’s best to space out sharpening and you need to repair it more, but in turn you get a lot more use out of it for taking care of it.
Edit: actually the whetstone could be used in crafting blades too, since, like in real life, blades need to be sharpened when they are made. It would make crafting blades and such more dynamic and interesting too.