Renovating a Slate Stone Water Feature

I was recently asked for a consultation from the current homeowner to see if anything could be done about this neglected water fountain.  The slate installation was at least 15 years old and the previous owner had not properly maintained it.  All the grout below the water line had degraded and turned to sand.  

Also, it was evident that a top surface-coating sealer had been applied, rather than a penetrating solvent based stone enhancing sealer.  This surface coating was peeling off in flakes.

Here you can see that all the grout disintegrated in the joints below the water line.  It appeared to me that the original installer did not use modified grout in between the slate tiles.  Modified grout contains ingredients to increase its strength.  This grout here appeared to be just sand and cement, which does not hold up well under harsh conditions.

In order to apply sealer, the stone and grout should be thoroughly cleaned first.  Here I am applying alkalai-based soap to the stone and grout.

After applying the soap, I used an industrial steamer to heat up the surface and clean the slate and grout joints.

Here is the re-grouted, cleaned and sealed water feature.

Here you can see that the colors in the stone are richer and deeper.  This is because I used a solvent-based enhancing sealer made specifically for natural stone.