Surface Preparation: Bathtub and Tile Refinishing Coating Failures
Coatings that are applied to improperly prepared bathtub and tile surfaces usually result in one thing; FAILURE. The question is was this failure the result of an improperly cleaned surface? Was there a contaminant left on the surface? Was there moisture left on the surface? Or was the new coating painted over the top of an existing coating that was already failing?
Surface preparation is the one of the most important steps for bathtub and tile refinishing. The purpose of surface preparation is to clean the surface and get the surface ready according to your coating manufacturer specifications.
When selecting a bathtub and tile refinishing coating manufacturer, it is critical that you follow their specifications to the “T” and use a system that is properly matched like the Topkote Reglazing System. If you wander outside the reservation your coating may not achieve the adhesion needed to perform as intended. When it comes to surface preparation, a failure to follow the specifications of your coating manufacturer is considered to be an expected premature coating failure.
When preparing a surface, various methods are used to ensure the surface is prepared to the specifications set forth by your coatings manufacturer. Surface preparation methods include stripping, hand/power tool sanding, etching and degreasing. The degree of surface preparation is dependent on the existing bathtub or tile surface and the coating system you intend to use.
Example, a tub that has been previously refinished like the one in Figure 1 shows signs of existing coating failure. In this scenario, the proper surface preparation method would be to remove the existing coating by means of a chemical remover, power tool sand to remove any residual coating not removed by the chemical stripper, degrease the surface, prime the surface using an adhesion promoter such as Topkote’s Crosslink III and apply finish.
Figure 1: Coating failure due to improper surface preparation.
When cleaning it is important that any “interference” material be removed from the surface or the coating may not properly adhere to the substrate. Grease, oil, dirt, existing coating that is failing are obvious interference materials. Less obvious contaminants may not even be visible, such as silicone or chalking.
Here are more examples that will help bring this topic into focus.
Refinishing over the top of a pre-existing coating that is failing
In a case where the surface was previously refinished like in Figure 1, the refinisher simply feather sanded around the failed area, cleaned the surface and refinished over the top. In this scenario, the coating will likely fail. Why? The existing coating had poor adhesion to begin with and the new finish will likely delaminate along with the old finish at some point and result in a FAILURE. The existing finish should have been removed.
Dew Point surface moisture
The bathtub or tile has been prepared correctly however; there is microscopic moisture on the surface. This microscopic moisture will not allow the coating to adhere properly which will likely result in a FAILURE. This microscopic moisture is caused by temperature and humidity and there is a point in which moisture will condense on the surface. It's called Dew Point. No coating should be applied unless the surface temperature is a minimum of 5°F above the Dew Point. Example: If air temperature is 70°F and relative humidity is 65%, the dew point is 57°F. No coatings should be applied unless surface temperature is 62°F. What should be done? A simple tool such as an infrared thermometer (Harbor Freight) can give you an idea what temp the surface is. If the temp is below what is should be; you can bring it up by using a heat gun (Harbor Freight). You can also use a solvent such as acetone to remove surface moisture however; it will also bring down the surface temp. Refer to the handy Dew Point chart below.
Chalking from acid etching
In this case, the refinisher used an acid etch (hydrofluoric acid) to remove the gloss from the bathtub. This is done to give the tub a proper profile to achieve a mechanical bond when using the etch & epoxy bathtub refinishing system. When it came time to remove the etch from the surface, the refinisher simply scrubbed with a scotch bright pad, rinsed the surface with water, dried & tacked the surface, primed using an epoxy primer and finished. Problem: chalking will occur on the surface from etching and create a barrier which will prevent the coating from adhering and result in a FAILURE. Solution: The surface should have been wet sanded after initial rinse then rinsed again.
Nowadays, refinishers are turning to the less hassle, no-etch, adhesion promoter method of refinishing like Topkote’s Reglazing System. With this method of refinishing, you skip the nasty acid etch process which saves time and after all, time is money!
The examples above show how important surface preparation is. If the surface is not properly prepared and you are not using a properly matched coating system, you will surely invite a coating FAILURE.