Flagstone is a material used for cladding exterior walls and surfacing patios and walkways. The stones are large, roughly flat pieces that are split from larger pieces of natural stone and then laid onto beds of gravel and sand or mortared in place against a concrete wall. As an outdoor material, flagstone does weather and get dirty over time. Sandblasting removes dirt, or spilt paint and mortar from flagstone by spraying abrasive grit (not actual sand) from an air compressor nozzle at high speed.
Rent a sandblaster from a home improvement or equipment rental store. If you have an air compressor, you can attach a sandblasting nozzle to the end of the compressor hose. Buy enough sandblasting grit for your project. Although it depends on the size of the flagstone area and the condition of the flagstone, you can easily use a hundred litres or more of grit for one project.
Thread the end of the compressor's outlet hose onto the base of the blasting nozzle. Turn the compressor on and wait for it to fill before proceeding.
Remove clumps from the abrasive grit by pouring it through a fine sieve as you fill the sandblaster's hopper to the fill line.
Surround the area of flagstone you will be blasting with dust sheets or tarps. The sand that ricochets off the stones and accumulates around your work area can be collected from the dust sheets or tarps and put back into the hopper to be sprayed again.
Put on your safety gear. You will need gloves, a dust mask, and tight goggles. Inhaling grit into your lungs or getting it in your eyes can cause serious discomfort and damage.
Open the outlet valve on the compressor to pressurise the air line connecting it to the sandblasting nozzle.
Slowly squeeze the nozzle handle as you point it at the flagstones. Move very carefully and slowly, avoiding contact with the mortar or sand holding the flagstones in place. No matter how careful you are, you will probably need to repair or replace this material.
Refill the hopper as it runs out. Continue blasting until the flagstones are clean.
Continue spraying after the last hopper of grit runs out to clear material from the nozzle and hoses. Shut off the outlet valve from the compressor and squeeze the handle on the nozzle until the air pressure in the hose is released. Shut down the compressor.
Replace damaged mortar or add sand to the areas between the flagstones that you accidentally sprayed.