You can't simply "touch up" compromised grout. You need to remove the bad grout to a suitable depth and regrout to complete a proper repair. I fix so many so called grout repairs where a company or a homeowner tries to do an "overlay" of grout on the affected area without the proper prep work. "overlaying" is the hack way to do grout repairs and should be avoided.Make sure grout lines aren't cracked, chipped, or otherwise compromised before treating with a sealant. If they are, touch up grout and wait the requisite 48 to 72 hours before sealing.
Some sealers need more time between sealer coats. For these sealers, wait 5 minutes and then wipe off the tiles to remove grout sealant that got on them.