The patients were subjected to four treatment protocols involving acidulated phosphate fluoride ( APF ) application followed by rinsing the mouth with water : ( 1 ) after 30 minutes ( APF-30 ) ; ( 2 ) after 15 minutes ( APF-15 ) ; ( 3 ) immediately ( APF-0 ) ; after cleansing the teeth with a cotton roll immediately following APF ( APF-cotton ) ; or ( 4 ) no fluoride therapy ( control ) .