Why won't my Limestone Stain Uniformly when I try to Stain it?

QUESTION

Customer has a pool deck with EW Gold limestone tiles. The style includes many pea-sized pits and 30-70% flat areas. The color varies in shades of light gray. We applied a stain and got a generally pleasing effect, however, some places seemed to lack penetration of the color. The stain manufacturer suggesting that grinding might open those areas up.

We ground a few tiles and reapplied stain but were disappointed to find that rather than accepting more stain, the newly-smooth areas accepted less color. While I had supposed that the stone was porous, the grinding shows that it’s not very porous inasmuch as the areas that have been grinded now have a sheen. None of the other tiles have such a sheen.

Our plan is to sand blast and or apply an acid etch to get rid of the sheen and hopefully provide a surface that will better accept color.

Here’s my question: Would you discourage either of these approaches?

ANSWER

ANSWER - It is not common to stain natural stone. There are enhancer sealers that applied to enhance the natural color of the stone.

Natural stone is just that.  Every piece of stone is somewhat different that another piece.  Even within one piece of stone the material makeup and the physical properties of one portion of the stone over another can be significantly different.   Limestone which is a sedimentary stone is made of organic material that is primarily calcium carbonate along with other minerals including clay.  Often one material within the stone is very dense where it can be adjacent to a very porous material.  So that is why the stain doesn't absorb in some spots is because it is a different material that is too dense to absorb the stain.

Obviously the acid etching and sand blasting will open up the pores of the porous portions of the stone, but it won't significantly affect the dense portions of the stone.  You have to be careful with acid etching and make sure you don't use too corrosive of an acid and rinse it well or it can penetrate into the stone and over time it will degrade the stone when it is activated with moisture.    Whatever you do, you should test your method on sample stones to make sure you can live with the result.

If you use stone then you should appreciate the natural variation of it that took millions of years to develop.  If you don't like variation then you should select a porcelain tile that looks like stone.

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