This sounds like something I would do... And I do lots of things that end of wasting huge amounts of time. Like many other people have said, this would suck to do and should probably be avoided.

Anyways, if you must do this, a high quality respirator is a must. You also must do something to control the dust. Even though it's outside this is critical to avoid putting yourself and others at risk. If you've never done this sort of work then you can't possibly understand how much dust it'll produce. You can't just wear a respirator while working. Unless you're very lucky with the weather, the dust will stick around and blow all over the place when it's windy. This could last days or weeks after the job is done.

You can grind dry with a pretty serious dust collection system or you can wet grind with a pneumatic grinder or a very well insulated electric grinder. I prefer the former, but if you must use the electric grinder to wet grind then plugging it into a GFCI outlet is essential...

But you really don't want to do this... If you just don't like the lines, maybe you could stucco the whole thing instead, I really don't know about that though.