The scholars have differed in regards to this. A group has said that there is no harm if the person who is fasting uses Siwaak before zawaal or after zawaal.
They have based their opinion on the large number of authentic ahadiith which talk about the virtue of siwaak
Others have said that it is makrūh for the person who is fasting to use Siwaak after zawaal regardless of the Siwaak he uses is a wet one or a dry one. They have based their opinion on the Hadith of Abu Hurairah (may Allah be pleased with him) in which it is mentioned that the smell that comes from the mouth of a person who is fasting is better in front of Allah than the smell of musk.
This smell (al-khuluuf) usually manifests after zawaal.
The Kuwaiti Encyclopedia of Fiqh