I have been away from masonry construction for over 30 years now. a lot of new products have been developed since i left the trade. That being said, you have to look a little further than any thing i might advise.
When was putting up chimneys, veneers and walls, most any type of paint sealers eventually cracked. Once a crack developed, water found its way into the masonry. It never seemed to find its way back to the outside from whence it had come. It just somehow found a way out thru inside walls. There were several "liquid" water repellants that soaked into the bricks. Those repellants, once into the brickwork shed water much more efficiently than the paint type repellants, and kept their repellant qualities much longer. Often, when a chimney or veneer that had been painted developed a leak, the solution was to sandblast and then apply a soaking type repellant, assuming structure and flashings were in good condition. The best waterproofing for brickwork was to use the best bricks and mix the mortar properly and in correct proportions, and then skill of the mason for the final application.
The above is just about how things were done up until 30 plus years ago. (I started in the trade in '49.) Your idea of a poly urethane is about products that were only just starting to appear back in those days. I don't know if the products we used when i was working with masonry are even used any more. I'm sure others will chime in here with more up-to-date knowledge.
