Durability and repair

Does anyone know if the loss of effectiveness of your armor and weapons scales evenly with damage to it, or if it's almost useless when it's at zero. If my weapon is 10% damaged, am I losing 10% of it's power? I ask because I always repair even if it's the smallest amount. I would rather wait if I could, but I guess I'm being all ocd about it in case it does scale.
Plus, does anyone find it odd there isn't a repair all button? I'm pretty sure Demon's Souls had one.

I don't believe your weapon an armors effectiveness scales with its health, but when you have the weapon or armor at risk icon I believe they become 50% effective.

I do what you do, I just indiscriminately repair everything.

"At risk" icon, huh? I have never seen that. Not surprising I suppose because I always repair. Good to know the game at least let's you know when it's a problem. I'm guessing it alerts you when it's 50% trashed then? I've never tested it, but I know there are probably some smart people who have.

I think its more like 10%. Grab a katana and just hit stuff for a while haha, you'll see it.

Basically what Timmy said, once your weapon hits 20%-10% of its durability you'll get a "Weapon at Risk" warning, and during this time it will only inflict about 50% of its normal damage. Only when the weapon hits 0 durability will you get the "Weapon broken" message, at which point it will only do about 10% of its normal damage. The only weapons you should ever worry about risking, let alone breaking, are weapons you've upgraded down the Crystal path, Katanas, or weapons with special R2 attacks like the Drake Sword since the special attack usually takes out durability to use.

I just tested this two days ago:

100% - 31% durability = 100% damage and reduction.
30% durability (weapon at risk) = 70% damage and reduction.
0% durability (weapon broken) = 50% damage and reduction.
Stability were not affected by durability.
Same for armors: 30% dur = 70% effect, 0% dur = 50% reduction and resistance, even poise is halved. The paladin set also cost 4,000 a piece to fully repair.

I haven't tried shields yet, but I'm curious if stability is unaffected like it were for weapons.