Sealing Indian sandstone

If it's mortar spots then you can use an acid based patio cleaner on sandstone. Just be sure to test it on any spare off cuts you have or a discreet area first, to make sure there is no adverse reaction with the stone. You can try it first watered down and gradually increase strength but i have used it neat with no problem. You may have to keep. Applying the cleaner to certain spots and gradually clean off with a hog hair brush. Hope this helps

If its rust spots it’s probably iron in the sandstone. Nothing will remove that. Better off changing slab otherwise it will annoy you for years to come. Mortar stains can be cleaned off yes with diluted brick acid or brick cleaner. Sealing stone will actually enhance the stains more. Resin or solvent sealer won’t stop it long term although will reduce the amount of water penetration therefore may slow the rust from becoming worse. Its a difficult question to answer. I have had odd rust spots on one or two slabs. But always changed them. I would contact supplier and complain. Its not normal to have 30% with stains.