The substrate must be clean and dry, ideally free of dust and small parts which may come loose (stable substrate). If this is the case, you do not need to prepare the substrate any further.  For highly absorbent building materials, it is best to dilute the acrylate sealant with water at a ratio of 1:4 and use it as a primer or undercoat. However, old joints must be redone, i.e. the old grout must be mechanically removed (scraped out) and cleaned before you can grout them again.