Repainting exterior sandstone
The best answer is to remove the paint. Get the sandstone dry - really dry, camped out in the desert for a day dry. I have found something like this, can convinced a block that gets no sun to dry/tan itself.

https://www.flickr.com/photos/some_guy/17000465161/

The stone will need to be at a temperature inside the bounds of the paint label.

Paint with something like

https://www.flickr.com/photos/some_guy/16381291883/

Paint with exterior paint of your choice (with paint you get what you pay for. Going to sherwin-williams is not throwing away money.)

As far as removing paint: Stainless steel brush of your choice. One for a grinder is not wrong, but it is easy to remove too much stone. Take note, removing paint from stone will really be removing the stone to which paint is stuck. Sandstone absorbs paint, you can not remove just the paint.