prosoco has a producct called H100 that can be used on Sandstone. their website is www.prosoco.com. It is a consolidation product.

I would not think a concrete densifier would be of any use on sandstone since there would not be calcium, or much of it, to interact with , since a sandstone should be mostly quartz based..although all these stone vary a bit, and your sandstone may benefit, but I wouldnt say it would benefit much. I f its been treated already with 3 coats of dry treat, I wouldnt think much would penetrate at this point