Brush-on Gels and Whitening Sprays are not effective and will not give you the results that you desire. This is due to a very simple reason. With brush-on gels and sprays, there may not be an effective active ingredient like Carbamide Peroxide and if there is an effective active ingredient, it is not held against your teeth for the length of time required to generate significant results. You will always need some sort of tray to hold the gel against your teeth.