We have recently been contacted by numerous customers who have been having problems with spot stains appearing on their sandstone patios.
This is a common problem, so we thought we should share why and how this issue sometimes occurs. The pictures in this post are supplied by customers showing the problems that are occurring.
Many of the imported sandstones, and some limestones and granites, are very porous. When fixing natural sandstone or limestone outside DO NOT SPOT FIX!!
They are not manmade concrete pavers and should not be treated the same.
The sandstone should be fixed to an even mortar bed and then each piece of stone back buttered for a solid fix…
This will help eliminate possibilities of staining from moisture and mortar.
Natural stones like sandstone are not as dense as concrete pavers and are full of microscopic capillaries. These capillaries can draw up moisture from mortar through to the face of the stone. The moisture dries out to leave a deposit of mortar residue and minerals within the pores of the stone. Sealing too soon after spot fixing can sometimes highlight these issues and also slow down the transition of moisture, suspending the cement residues within the stone.
The introduction of moisture from rain can also further highlight these issues. This is because the moisture collects and dwells within the voids between the mortar spots underneath the stone. As it dries out the moisture rises, carrying more minerals from the sand and cement to the surface of the stone.
Most sealers for outside use are breathable and allow microscopic transition of moisture, so any trapped moisture is still able to dry out should it become trapped beneath the stone. Sealing does not cause the issues associated with spot fixing, nor should it be considered a method for covering up the stains should they occur.
If your stone pavers have been spot fixed, then we would suggest not sealing them for at least three months after installation. This should allow for any mortars to full harden and any released minerals to completely dissipate. Clean surfaces well using LTP Power Stripper and leave to dry for 24 hrs. Test the sealer on a small area before general application to make sure that the mortar spots are not highlighted and, after a few days, make the test area wet and allow it to dry out. If staining does not occur the rest of the surface can be sealed.
If your stone has been fixed correctly into a solid bed then we would suggest a drying time of two weeks before sealing. This should allow enough time for all mortars to fully dry. Solid fixing will not allow any rain water to sit beneath the pavers and dwell. Clean surfaces well using LTP Power Stripper and leave to dry for 24 hrs. Seal using an impregnating sealer and apply as many coats as necessary to completely saturate the stone.
When sealing using an impregnating sealer it is important to remember that the solution needs to fully absorb into the stone without drying on the surface and do not apply in direct sunlight. If sealer is still visible on the surface after 45 minutes saturation has been achieved and any excess should be toweled off. For best results, allow at least three full dry days after treatment.
LTP IMPREGNATING SEALERS SUITABLE FOR EXTERNAL USE
CLEANING SANDSTONES & LIMESTONE WITH SPOT FIXING ISSUES
Use: LTP Power Stripper
Apply undiluted to the surface of the stone and agitate using a black emulsifying pad. Add a little water and agitate again. Leave the solution on the surface of the stone for 15 minutes and agitate again. Rinse well with water. Repeat until the stains are no longer visible. Leave to dry for a minimum of 24 hours before sealing. Before re-sealing the whole area test the sealer on a small area before general application to make sure that the mortar spots are not highlighted. Leave to dry and after a few days make the test area wet and allow it to dry out. If staining does not occur then the rest of the surface can be sealed.
CLEANING MORTAR STAINS FROM SANDSTONES AFTER POINTING
Apply un-diluted onto the surface of the stone and agitate immediately using a black emulsifying pad to release the mortar stains from the surface of the stone. Using a sponge and water, rinse well immediately after use and change rinsing water after every five sq/m.
MOULD, MILDEW AND BLACK SPOT
Spray LTP Black Spot & Algae Remover onto the surface of the stone and leave for 1 – 5 hours depending on the level of soiling. Using a black emulsifying pad, agitate the surface and rinse well with plenty of water. Repeat until all organic matter has been removed. Re-seal surfaces once thoroughly dry.
This information is offered without guarantee. The material should be used so as to take account of the local conditions and the surfaces to be treated. In case of doubt, the product should be tried out in an inconspicuous area.