I wouldn't sand blast them as this will take a fine layer of sandstone off, and over time doing this again and again will just spoil the finish. What I used in my sandstone wall and patio was a big bowl of boiling water with bleach and Fairy Liquid. Scrub this in with a hard hand brush and wash the residue off with a hose pipe and it will then look new for the next year.
If you type the key words of your question into our search box to the left of the site you may find the answer is already posted or is in the DIY projects section of the website. Every post goes through a monitoring process and using the search box may speed up your answer.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!