Sandstone-Fixing & Sealing Outside.


We have recently been contacted by numerous customers who have been having problems with spot stains appearing on their sandstone patios.

Sandstone Flags with Spot Fixing Issues.

Sandstone Flags with Spot Fixing Issues.

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.


Understanding Sandstone

Many of the imported sandstones, and some limestones and granites,are very porous. If fixing natural sandstone or limestone outside DO NOT SPOT FIX!!

They are not man made 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 illuminate 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.Img3701 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 and the moisture rises carrying more minerals from the sand and cement to the surface of the stone.

Most sealers used 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 3 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 3 full dry days after treatment.








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.


Use: LTP Grout Stain RemoverGrout_Residue_Remover_Pack2

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 5 sq/m.


Use: LTP Mouldex

Spray LTP  Mouldex onto the surface of the stone and leave for 1 hour. 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



Natural Stone Tiles – How to Fix & Seal to Avoid Discolouration?

Installation of natural stone tiles can cause discolouration?

Poor tile installation and fitting of natural stone surfaces is often the cause of discolouration of the surface. Natural stone tiles should always be buttered on the back and then laid into an even bed of adhesive. Spot fixing often causes issues with discolouration of the tile surface.

Stone floor that has been spot fixed

Natural stone floor installed using spot fixing. This has caused discolouration of the tiles.

What is spot fixing?

This is a method of tile installation and slab laying that involves only placing enough grout or cement at the corners and centre of the tiles, in order that it is laid level. This process will leave voids between the adhesive, tile and the sub floor. These voids will be the root cause of the issue of staining and discolouration.

What causes the discolouration?

Tile installation requires the use of moisture with in the adhesive used to fix the tile. When using spot fixing to install tiled surfaces this moisture can collect in the voids between the tile, adhesive and sub floor. When this occurs, minerals from the adhesive or concrete sub floor become activated. These minerals can then travel through to the face of the stone leaving a ring mark or staining effect. Each ring outlines where the spot of adhesive is under the stone.

Interior floor showing spot fixing discolouration

When spot fixing is used the appearance of interior floors can be significantly effected

The difference between Natural Stone and Concrete?

Spot fixing natural stone is not the same as spot fixing a concrete paver. Concrete pavers are in general much denser with fewer larger capillaries. Fixing concrete to concrete with cement does not normally result in the same staining issues. However, when fixing a natural stone paver it is important to butter the back of each slab with mortar before laying them into an even bed of mortar to avoid staining.

Natural Stone Patio with Spot Fixing Discolouration

Spot fixing is also an issue when natural stone tiles are laid outside

Removing spot fixing discolouration

Removing staining associated with spot fixing is very difficult. This is because it is not just on or near the surface, but right through the stone. Scrubbing the surface with a residual grout or efflorescence remover such as LTP Grout Stain Remover may help to lighten the appearance of the ring marks, but it does not always completely eliminate them. It is also often harder to remove the marks if the surface is sealed when laid.

Acid based cleaners such as LTP Grout Stain Remover can also affect acid sensitive surfaces such as limestone or marble, however if the they are rough cut or tumbled the contact with an acid makes little difference to the overall appearance provided that it is applied evenly. Stones such as sandstone and slate are not normally affected.

Quite often the only effective method of resolving this issue is to rip up and replace and then complete tile installation correctly following your adhesive suppliers recommendations.

Laying natural stone tiles correctly

When laying natural stone tiles correctly we would suggest sealing the stone with LTP Mattstone, LTP Mattstone H20 or LTP Colour Intensifier & Stainblock, depending upon your surface type and desired finish. These are impregnating breathable sealers that will help to enhance and protect the stone and at the same time allow the microscopic transition of moisture.

PLEASE NOTE: Always follow the detailed instructions on the product labels. Protect stone surfaces from contact with acidic substances. Do not use abrasive materials on honed or polished stones. Avoid using normal detergents or strong bleach cleaners on sealed surfaces as they will gradually remove the protective seal.

At LTP we believe in helping to offer solutions and this is why we offer a no obligation LTP TECHNICAL HELPLINE, so please give us a call on +44 (0) 1823 666213 or email and we will be happy to help you.

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