Hi. The ideal is always to enlarge the cracks and just repair them. For this you need a grinder and a concrete disk. Make them at least 3 cm wide or 1.5 cm on each side of the crack. You can make them wider if you want and their depth should not be less than 3 cm. Once all of them are widened, you should clean the powder and loose parts very well and apply an epoxy monolithic adhesive called SIKADUR 32 Gel. As for filling the cracks, it uses three sand, one cement and water repellent. Do the mixture and respecting the times of the adhesive fill them and flatten neatly. To give a good finish, finish filling with fine sand and the same cement and water repellent aggregate. Let dry very well (at least and at least a week) and go. You can now paint the cracks repaired with the completion of the sink. If you do a good job you will never have problems in that area or in those repairs. We have done it many times in many pools with 100% success. regards https://per.sika.com/dms/getdocument.get/739ade70-3a45-3218-bb81-d35516c94976/HT%20-%20Sikadur%2032.pdf https://arg.sika.com/dms/getdocument.get/fbdfe92b-ca8e-3763-b649-6052b86993ab/ht-Sika1-es-ar.pdf