Sandstone is the softest stone used in construction; so soft that you can actually pick away chunks of it with your hands. You would think this characteristic would render it useless in building, but it is actually a prominent building material. You do have to take into consideration its brittle nature before you attempt to paint it. Protection and delicacy is required and, if you achieve that, you might get the perfect coat on your sandstone.
Experiment with your surface of sandstone to ascertain whether it has already been protected from the elements. Pour a teaspoon of water on the stone. If it is immediately absorbed, you have to treat the stone; if it rolls off, the sandstone has already been protected and you can carry on to the painting phase.
Paint the wall with a coat of sealant if, after Step 1, you realise the sandstone needs protection. Allow the sealant to dry completely.
Clean the sandstone of any dirt and debris. There is no use painting on sandstone --- or any surface, for that matter --- if there are layers of debris blocking the coat. Use a cloth to remove any loose particles; do not use sandpaper, the stone is too brittle and you will damage the wall.
Purchase or rent a paint-spraying gun to paint the sandstone. As the surface you are working on will presumably be very large, it is the most effective way to apply the paint. Sweep the paint gradually across the wall, ensuring even coverage. Another benefit of a paint gun is that the paint will enter the crevices of the stone.
Apply a coat of neutral-coloured primer first and allow that to dry. Load the spray gun with your colour of choice for the sandstone and apply that.
Use a spray-can of clear-coat to apply to the sandstone once the coats of paint have dried. This provides your painted sandstone with additional protection.