Pre-Sealing - Don't do it!
Pre-sealing is sealing the floor or wall surfacing material before it has been grouted.
The sealer then keeps the grout from sticking to the face and makes clean up easier.
This is done because typical grouting involves packing the grout into the joints by smearing it over all, or part of, the material face. Even with a grout bag applying grout only into the joints, residue is still left behind an inch or more from the joint as in this picture.
No grout barrier was used for this installation, thus grout stains remained. It was cleaned with an acidic liquid cleaner. However, that should not have been necessary.Why you should not pre-seal!
Although "pre-sealing" works fairly well in accomplishing the short range goal of easier grout cleanup, this technique increases the risk of other problems to an unacceptable level. These risks are:
The Focus Is Wrong.
The focus of a "pre-seal" sealer application is to not spill the sealer all over the floor or bench on which the ungrouted materials are sitting. This results in the first, and most important, application of sealer being done in exactly the worst possible way. Specifically, the sealer is being "painted on" in a thin coat instead of deeply penetrating.
If you are using a "penetrating" type sealer - it prevents the proper penetration of subsequent sealer applications.
If you are using a "coating" type sealer - the succeeding sealer applications can only bond to that first, poorly adhered first application which is at risk of losing its bond to the tile surface. Thus, all the sealer layers could peel off down to the surface.
The "painted on" sealer has been applied in labor intensive, multiple applications that increase the job cost of labor time for the contractor, or work effort for the do-it yourselfer. Painting on a pre-seal layer also increases the risk of streaking, and/or other applicator marks from brush, rollar, etc.
The Floor Is Now Unevenly And Partially Sealed.
After the tile are installed and grouted - the tile is all or partially sealed, and the grout joint is not. Now, it is necessary to apply more sealer for the grout, and possibly, for the tile. This may result in more sealer on the tile and less on the grout joints than is desirable. The pre-seal application will interfere with the designed penetration and adhesion characteristics of the final sealer.
Achieving A Good Bond Of Sealer Layer To Sealer Layer
Almost always the surface will be sealed after completion. The first sealer that was used as a "pre-seal" may not be chemically compatible with the finish sealer. The first sealer may repel the finish sealer because, of course, all sealers are designed to inhibit penetration of liquids. This is also the reason a sealer should never be used over, or under, any other sealer on new jobs, or when Re-doing a Previously Sealed Surface Cleanliness
In order to achieve good bonding between sealer layers, the first sealer layer needs to be perfectly clean to receive succeeding sealer applications. In the case of pre-sealing, the first sealer layer has obviously been abused, and contaminated by the grouting and cleaning processes.
Potential Stain Trapping
Moisture and/or efflorescence (white, powdery stains) can be trapped under a painted on pre-seal coating and create difficult cleaning situations that require stripping off the sealer.
Creating An Unknown Risk For Followup Sealing
The tile installation contractor who has pre-sealed may not be the same individual doing the finish sealing. A different individual doing the finish sealing may be faced with the same risky situation as described in Re-doing a Previously Sealed Surface - and may not even know it.
Any "pre-seal" application will invariably drip onto the edges of the tile. Wherever this happens it can interfere with the bonding of grout to the tile edge. Increased Lime Pops
In the case of "Mexican Tile", the presence of a painted on pre-seal layer on the newly set tile will slow down normal water evaporation from the setting bed. This can increase the quantity of water available to infiltrate lime pockets, thus increasing the risk of lime pops. The Conditions When You Might Need To "Pre-Seal", Despite Everything Above!
With easily stained surfaces, like some of the light colored Terracotta tiles, test first. In that case the only alternative is to pre-seal. However, apply the sealer using the same proper technique described on the container label, as best possible under your circumstances. This won't be easy, but do the best you can to get enough liquid on, and in, the surface to accomplish the job properly. If you must pre-seal, apply the sealer according to the product label, but in such a way as to keep it off the edges that will need to bond with the grout. Then grout anytime after the sealer has dried to the touch. Use the normal grouting technique and let it dry completely before applying another sealer application to the tile and the grout. When the sealer is fully cured per the label directions, then a "sealer protector" can be applied.