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How to clean & maintain stone mullions

How to clean & maintain stone mullions

How to clean & maintain stone mullions

If you wish to clean and maintain interior natural stone mullions, we recommend Floorseal Stone Pro. We have created this handy guide to explain what affects your stone mullions, why it happens, and what you can do to clean & maintain them.

If you wish to view individual product pages for pricing & further information, please click on the product titles highlighted in blue.

What is growing on my interior stone mullions?

If the problem you are noticing is a black speckle it will probably be organic. Usually a black mould or perhaps a white mould (which can look almost like a fur).

Mould often occurs on natural stone which is cold and usually a little damp. Areas with less, or little airflow, will allow mould to grow.

Mould can spread from the stone onto plaster, wood or paintwork, adjacent to your stone mullions.

Cleaning stone mullions is an ongoing process

How to remove mould from stone mullions with Floorseal Stone Pro

Typically any mould will respond to a treatment of Floorseal Stone Pro. This product will remove organic contaminations like black or white mould. Stone Pro is a perfect cleaning solution for stone mullions, but also garden ornaments, headstones & statues etc. Stone Pro is suitable for use on natural stone like sandstone, marble, granite, limestone, slate, lime, plaster & more.

The product comes in a 5-litre container and has a useful sprayer included. Dilute the product 1:1 with water – A mixture made up of 50% water and 50% cleaning solution and spray onto the affected surface.

How to apply Floorseal Stone Pro

Apply Stone Pro to the stone by spraying over the surface, followed by an immediate overspray. The solution will rapidly remove general moulds which affect interior stonework. You can rinse the surface with clean water, or for light applications in awkward areas, simply leave the product to dry.

Try not to apply to metals such as window catches, locks and frames. If the product does come into contact with metals, wipe off with a damp cloth (or rinse) then buff dry.

Why is mould growing on my interior stone mullions?

Mould will grow on stone mullions because they are a little damp, usually because they are a cold stone in a warm room. Mould is normally the result of moisture appearing on the stone. Classically stone mullions do not display mould growth during warmer summer months, but during the autumn and winter mould starts to appear.

The reason for this is that the stone often extends from the outside of the windows into the house. Stone mullions are often quite cold compared to air temperature within the house. In these conditions, moisture can condense onto the surface of the stone. Mould loves damp conditions, coupled with the warmth of the room, it triggers mould to grow and spread.

Mould will grow on stone mullions because they are a little damp usually because they are a cold stone in a warm room

Can I seal my stone mullions, will it prevent mould growth?

If your stone mullions are ‘mould-free’ in the summer and not in the winter, sealing will probably not help. This is because the mullions have not become damp from moisture penetration from the outside surface due to rain etc. It is just a byproduct of condensation forming on the interior surface. Damp from condensation may or may not be obviously visible, but if it is there mould will often grow. So sealing the exterior face of the mullions may have no effect as the temperature of the stone is still the same on the interior side.

It is not possible to seal the stone from the interior side of the mullion against moisture due to condensation. Usually, you will need to remove mould as it forms. 

You should still check that the seals (such as putty or silicone) are intact on the outside face of your windows to rule out moisture penetration rain & damp.

In conclusion

Floorseal Stone Pro is a deep cleaning product for natural stone which has organic growth noted on the surface. Typically this will be black mould formation on internal stone mullions.

Floorseal Stone Pro arrives complete with a small container and spray maker. Dilute the solution 1:1 (one part cleaner to one part water) and spray over the surface. Rinse or wipe away the product or for minor issues and awkward areas allow the product to dry on the surface. Avoid contact with metals, wipe away or rinse metals if they come into contact with the cleaning solution.

Please click here to view our other handy ‘how-to guides’. Or click the product titles above highlighted in blue for pricing & further information.

The information contained within this article is not exhaustive. If you need further information please contact Floorseal on (01484) 861461 or email us at – our web address is