after zawal it is makrooh to make siwaak, unless one wishes to remove the taste or smell of sleep from the mouth. & one must make sure not to swallow the saliver because anything that mixes with ones saliver breaks the fast when saliver is swallowed, even the colour on cotton thread if it comes out when one wets it in the mouth would break the fast. this is for the shafi' madhab, I cant find rulings dealing with atar, but I can tell you that it would not break the fast because you cant break your fast through the skin under normal circumstances.