Convenience sampling will be used, in which patient will be recruited from General Dental Clinic (Sapphire), Faculty of Dentistry, Universiti Teknologi Mara. The total number of participants needed is 132. At baseline, during the screening and examination stage, apart from ICDAS score and the decayed, missing, and filled surface (dmfs) index to record each child's caries experience, recorded status will also include baseline bitewings of posterior teeth and DIAGNOdent reading (triplicate reading will be carried out to achieve accurate value) for all lesions. Periapical radiograph of anterior teeth will only be taken to confirm no pulp involvement when depth of lesion is questionable. Five tooth surfaces (buccal, lingual, mesial, distal and occlusal) will be examined in each posterior tooth and four tooth surfaces (labial, lingual, mesial and distal) in each anterior tooth. A carious lesion will be recorded as active if softness is detected upon gentle probing. If the lesion is hard when probing, it will be classified as an arrested caries. Screening and oral examination will be conducted on a dental chair in assigned clinic at Faculty of Dentistry, Universiti Teknologi Mara, primarily through careful visual inspection with the aid of a World Health Organization (WHO) CPI probe and a stainless steel/disposable front surface dental mirror by the main researcher. The main researcher will record each patient's caries status on a separate paper-based trial record at the beginning of the study in which she will not have the access to Integrated Dental Record Management System (IDERMS) in order to maintain blindness. Before conducting the study, a training and calibration session for oral examination (ICDAS and DIAGnodent utilization) is planned for the main researcher with one specialist in dental public health and one specialist in paediatric dentistry with the aim to achieve the kappa values of inter and intra-reliability ≥0.8.

Next, after the screening and before consent, if the child is eligible for the study, parents or guardians will be provided with information about SDF and NaF, follow-up instructions, oral hygiene instruction and diet education which will use video and pamphlet. Parents and guardians who agreed to participate in the study will be given questionnaire which include children's demographic background, oral-health related habits, and Malay- ECOHIS together with the information sheet and consent sheet. Appointment for first topical fluoride application will be given within one week from baseline oral examination day.

On the first appointment day, the allocation of patient between 38% SDF application and 5 % NaF application is randomized using sequentially numbered, opaque sealed envelopes (SNOSE) procedure (Doig \& Simpson, 2005). Block of four and six will be used to ensure that similar numbers of patients are enrolled into each group and to ensure the prediction of allocation sequence impossible. The order of the blocks will be determined by flipping a coin. This will be carried out by a dental officer/postgraduate student who is not involved with the study. He/she will keep the piles of blocks and when participant comes for the intervention, he/she will ask the operator to select one envelope from the first pile of blocks according to allocated sequence and will prepare the treatment selected. After first application is completed, he/she also will be the one who will update the Integrated Dental Record Management System (IDERMS) on which treatment has been used for the respective participants in order for him/her to prepare the same treatment for the 1-month follow up. The topical fluoride applications will be carried out by the other three researchers who are not involved in the screening and assessing of the lesion activity. Before the study, they will be trained by a paediatric dental specialist who is not involved in the study for the clinical application procedure. This is a double-blind clinical trial where the main researcher is not aware of which treatment the participants will be receiving, the other researchers are not aware which treatment they are providing and participants are not aware of which treatment they are receiving.

Participants will be called again for second application of same treatment they received on the first appointment after one month. Final appointment follow-up examination will be given at three months after first application of the topical fluoride. Same examiner will conduct the follow-up oral examinations using the same equipment, procedure and diagnostic criteria as those used in the baseline examination. Another questionnaire inclusive of parental acceptability for SDF treatment on their children and OHRQoL (Malay-ECOHIS) will be distributed to the same parents or guardians and will be asked to complete the questionnaire survey. After study is completed, the participants will be assigned back to undergraduate students for definitive treatment.