Oren:

The only "spray on" grout sealer I've ever heard of is a scam.

When homeowners discover that the price of their new bathroom tiling doesn't include the cost of sealing the grout, they want the grout sealing to be included in with the price quoted for tiling.

So, to keep the homeowner happy, the contractor will seal the grout using a spray bottle and a rag. He sprays the grout sealer on with one hand, and immediately wipes it off the faces of the tile with the other hand. The thinking here is that the grout lines are recessed whereas the tile faces stick out proud of the grout lines. So, theoretically, the rag only wipes the grout sealer off the tile faces, not out of the grout lines.

And, if you believe that, I have some ocean front property in Nebraska you may be interested in purchasing from me.

Really, the truth is that fresh grout is highly alkaline because like all Portland cement based materials, it has hydrated lime in it. Mildew won't grow on grout for at least two years while the grout's alkalinity subsides. So, when a tiling contractor tells you that you don't need to seal grout, it's a half truth. The contractor knows that no mildew will grow on his grout during the first two years of the grout's life because of the high alkalinity of that grout anyhow. And, since his guarantee expires after only one year, from his perspective, it's not necessary to seal the grout since it doesn't need to be sealed to avoid mildew growing on it within his warantee period.

And, of course, a tiling contractor will tell his customer it's not necessary to seal grout just to avoid having an argument with the customer as to whether the price quoted for the tiling should or shouldn't include the cost of sealing the grout.

The only grout sealer I use is an acrylic film forming grout sealer that I paint on with an artist's "script" paint brush. I've never heard of a "spray on" grout sealer other than the scam described above. By rights, homeowners should seal their own grout, but most never do, with the result that their bathrooms never "wear out", but "ugly out" instead.