Originally Posted by
Rifokelt
Armor reduction /=/ armor penetration.
Expose armor/Sunder armor/Feary fire/Shattering Throw reduce a target's armor by some amount. Armor penetration factorns in after armor reducing debuffs have been applied to the armor of the target.
In practice, armor reduction retains a lot of value even if one has capped himself to the arp cap of 1,400, since you don't fully ignore a target's armor past a certain amount.
TL;DR They don't stack per say, but rather work together.