Any non-critical hits would first be taken from the armour's HP. Crits do two dice of damage - one to armour, one to the character's HP. I've been thinking recently about having two kinds of crits - one for 20's on d20, and one for 6's on damage dice, so this would be for rolling max damage. More on this in a later post...
Just like a character, the armour would be able to be "healed" - but in the case of armour, it would require time and tools.
This would give an advantage to the lighter armours - as the armour gets heavier, so does the repair kit! At the high end, you're not going to be making much in the way of repairs to your plate without a set of hammers and an anvil - hardly typical adventuring gear!
Thinking out loud here, but let's toss some numbers together:
Armour as HP
Repair time (max HP without proper tools)
Max HP with lighter tools
Scissor, needles, thimble, strong thread
Sharp knife, scissors, heavy needles, awl, thimble, strong thread
As Cloth, plus:
Wire, wooden core, wire cutters, pliers
(Optional - mechanical wire winder, specialised pliers and snips, premade rings and rivets)
Plate and Chain
All of the above, plus:
Several hammers, heavy snips, steel plate, files, anvil
(Optional: spare plates, straps, buckles, etc., planishing stakes, dishing stump)
Without proper tools, you can't repair the armour.
If you have the proper tools for a lighter armour (i.e. have Cloth tools but Chain armour), you can repair the armour to the level shown in the last column.
If you have the optional tools, knock 25% off the time.
All of these times are for field repairs - expedient, but probably not pretty.
Unbalanced weapons would always do 1 point to the character's HP, even on non-critical hits.
I thought of this after re-reading this post on Untimately: