Recurring Spots in Flagstone - Can I seal this?

[Uglier]

We had several dark spots on our new flagstone fireplace hearth, I washed with diluted muriatic acid and that seemed to remove them 100%, but after a week they have returned just like they were!  Would a caustic clean more deeply? If I sealed over these right after cleaning I'm afraid the spots would come right through the sealant.

[Mark D.]

How is the flagstone adhered to the hearth?

[Uglier]

The stone is about 2 inches thick and its mortared to several courses of bricks below.  The spots are random in size and take up about 5% of the total area, they came back exactly where they were before. One possibility I thought of is that it is coming from the antifreeze chemical they had to add to the mortar when they built this in the freezing weather last December.  I can't remember what its called, but it comes in an orange pail.  The bricks don't seem affected by it though.

Thanks, Dave

[Mark D.]

You can make a caustic solution with caustic soda & water, or you can use Poultice, or for that matter you can make your own poultice with plaster of paris powder and Coleman liquid lighter fluid. If you do this after placing the solution on the stone allow it to dry until it is a powder and then brush off and clean the stone if necessary.

Whichever way you go I would test it first in an inconspicuous place to make sure it does not stain the flagstone! After a week of the stain not coming back, wait another week (just to make sure) and then seal the stone with an impregnating mineral based sealer. Think HYDREX!
http://www.defusco.com/tenax-impregnator-and-sealer-liter-p-2095.html

Mark