Additional damage timing, when not specified

[GaleonBlazer]

I was wondering: when it's not directly specified onto the card, do I add additional damage before or after applying Weakness/Resistance?

Citing Compendium LV.X, which is not crystal clear on this passage:

Overzealous
Q. Machamp's "Overzealous" Poké-POWER adds 30 more damage to its attacks; is this 30 done before or after Weakness/Resistance?
A. Overzealous adds +30 to the attack, but does not change the attack's base damage (for purposes of other Pokémon copying Machamp's attacks). It is figured in at Step , when you add damage effects on the Attacking Pokémon. (Jul 1, 2004 PUI Rules Team)

How does the rule about additional damage generally work?
How about if base damage was modified, instead?

Thanks!

 

[PokePop]

All damage is done at the same time. Unless explicitly stated to do that damage at a different step in the attack.

The question you quote talks about an effect on Machamp that would not be seen by another Pokemon copying that attack and doing the damage themselves.
I don't recall why it's even mentioned in response to this question! It seems to be beside the point.

It does change the Base damage for Machamp doing the attack, just not for another Pokemon copying that attack for their own use.

It's a 5 year old ruling on a specific attack combined with a specific Power. I wouldn't try to use it to make a general case.
If you have a question on a similar more recent attack and/or power, best to ask about that directly.