subtle tooling/etching of flagstone

Hello,
I'm looking at building a fountain/birdbath thing that will possibly use flagstone for the watercourse. I'd like to subtly tool or etch or whatever a slight depression in the stones to help the water flow in the right places and continue to the reservoir rather than just flowing all over the place and onto the ground.
I'd like to do a more subtle job than my talent w/a rock chisel would end up with.
Anyone done this before?
Can I use muriatic acid slowly etch away a depression in the stone? Or would that just take forever. How about one of those drill mounted sanding drums w/some sort of diamond grit sandpaper?
I guess one other question. Does the porosity of the flagstone make this whole idea a non-starter? I.e would the flagstone end up constantly absorbing all the water and sweating it out the bottom so that the reservoir would always need filling? If that's the case is there some sort of invisible coating that could be applied, I guess like the stone sealers you use for countertops and stone floors that would eliminate that problem.
Just looking for pointers before wasting a bunch of time trying things that won't work.
thanks ml
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On 7/9/2008 9:54 AM kzin spake thus:

Don't know for sure; just guessing here, but sanding or grinding would probably be much more visually evident as the surface would get scuffed up. I'd go for cutting tools, like chisels and gouges. (A gouge, basically a curved chisel, would be my first choice).
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sand blaster. use silicon carbide or aluminium oxide as a blasting material. this would get you a depression with no tooling marks.
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