Hello XXXXXXXXX,
Welcome to Just Answer.
I would definitely seal the grout and wait at least 48 hours after grouting before applying the sealer so the grout has time to cure. I do not think that the Johnson's Plaza Plus will work because it is not recommended for glazed ceramic tile. I would purchase a typical grout sealer at your local tile store or home improvement store. Johnson's is typically used on porous tiles (unglazed ceramic tile, granite, brick, slate, etc.) to give a high gloss finish on entire floor.
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Best of Luck, Brian
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Architect
Licensed Architect- 12 years, L.E.E.D. AP, Former Remodeling Contractor- 10 years