Thunder Damage Negative Id

Does the thunder damage negative ID on Ivory and Silver make it so that you do less damage on spin attack? Or does it only make a difference if you use thunder powders on the weapon?

 

I'm not sure but I question that myself too

 

I see no reason why it shouldn't affect both.

 

Negative element damage ID means, that the damage that you normallt would deal, is deducted by that percentage.
Say I would normally deal an average amount of 150 damage in fire per hit. I then put on a peice of equipment that have negative 15% fire damage. I would then only deal 150*0.85 = 127.5 fire damage per hit.
If there are two pieces, the percentages will be added together. 15% + 6% = 21%

Any elemental damage ID will not effect anything other than the damage dealt with that element.