Does the ability Overcoat have any other effects in battle other than what is specified by the flavour text?
>Protects Pokémon from Weather Effects and Powder moves such as Sleep Power, Poison Powder etc.
So, an Overcoated Pokemon can protect themselves from not only weather effects, but also the evil Powder Moves. >:D
Hope I helped. :) Source
Overcoat protects the Pokemon from powder and spore moves and only weathers that hurt Pokemon.That means that it protects Pokemon from only sandstorm and hailstorm.
Source: http://bulbapedia.bulbagarden.net/wiki/Overcoat_(Ability)
Hope I helped!