Terracotta floor regrout

[Pipnosis]

We have used a grinder to take a millimeter or so off our floor as it was 40 years old, filthy, damaged and yucky. The tiles look great now and are natural unglazed terracotta. However some grout is now missing and now we need to regrout. I have bought Lithofin "stain stop w" as a sealer, but not sure how to proceed next. Can I put the sealer on now and regrout after? I want to avoid grout haze. The tiles are now a natural matt pale pink clay colour. The surface is very porous. How can I add grout without leaving a mark on them. The old grout is very dark brown and thin 1 or 2mm. Total 60sqm, of 20cm tiles.

Any help greatly appreciated.