AC penalties... apply to only regular or all forms of AC?

[Archimedes The Great]

I was looking at the Reveal Weakness ability from the Wizard's Void arcane school powers.

The text only states a "penalty to AC". Initially, it looks like this applies only to regular AC, but what if the target has no actual armor bonus or natural armor bonus? Then it would do nothing. Looking into it deeper, now makes me speculate that it would apply to all 3 types of AC.

I know there are rules for types of bonuses and where they apply, but I'm not certain on penalties.

Thanks

[Kayerloth]

As far as I know they are penalties to AC, not a penalty to enhancement bonus (or other bonus types) to AC.

And all creatures have an AC.

And if for some reason the creature has no AC ... I'd say there's nothing to apply the penalty to.

[Syries]

if you have a penalty to AC it applies to normal, touch, flat footed, and CMD unless otherwise specified.

[Derklord]

Archimedes The Great wrote:

what if the target has no actual armor bonus or natural armor bonus?

Armor bonus is not the same as the Armor Class (AC). Armor bonus is, well, a bonus (to AC), dito for the natural armor bonus, and all others (dodge, deflection, etc.).

The CRB says "Your AC is equal to the following: 10 + armor bonus + shield bonus + Dexterity modifier + other modifiers." CRB pg. 179

For touch AC, the CRB says this (same page): "When you are the target of a touch attack, your AC doesn’t include any armor bonus, shield bonus, or natural armor". As you can see, it's only a modification of the above calculation, not a separate thing. Technically, there is no distinct "touch AC", you simply don't add all the bonuses against a touch attack.
Flat-footed is similar - according to the table on pg. 195, flat-footed is an armor class modifier (+0 but with the additional effect that "The defender loses any Dexterity bonus to AC."), thus once again not a separate thing. In other words, there is no discinct "flat-footed AC", you simply don't add all the bonuses when you are flat-footed, or against attacks that make you count as flat-footed.
Stat blocks merely list TAC and FFAC for ease of use, just as they list other claculations absed on general rules.

Basically, your AC is individually calculated for each attack, always from the same base, and thus a penalty to AC always applies. I don't think there even are penalties to AC bonuses, usually things lower the existing bonus (cf. the Armor Piercer Rogue Talent).

[Anguish]

Penalties to AC also explicitly apply to CMD.

[Derklord]

Oh, right, forgot that. That's because of a special line, as CMD is seperate from AC: "Any penalties to a creature’s AC also apply to its CMD." CRB pg. 199