Sandblasting and power washing are two of the most popular techniques used when converting farmhouses and property to achieve a “rustic” or “old-style” look. However, as conservation expert Joe Magro Conti explained, these techniques do more harm than good.

Advertisements to renovate houses using sandblasting, power washing and stone hacking abound but are the wrong tools for the job, said Mr Magro Conti.

After the stone is cut in the quarry, it builds a protective patina over the years and turns a certain colour over time.

“The colour of the stone depends on the quarry it came from,” he explained.

The patina is usually formed by the growth of small lichens and exposure to the elements. “Although lichens are not aesthetically pleasing, they do not harm the stone,” he explained.

However, most people do find its appearance displeasing and do their best to remove it.

Mr Magro Conti explained that not all patinas are healthy. “The dark black layer old stone buildings acquire in city centres damages the stone,” he said.

Plastering concrete on the walls does not solve the problem either, he added. The cement used in Malta is clay-based and is high in sodium chloride.

The salt is absorbed by the porous stone, which becomes corrosive when it comes into contact with water. As a result the stone crumbles away from the layer of cement. However, instead of tackling the problem in the correct way, most people simply plaster another layer of cement over the old one.

Before cement was used as extensively as it is today, houses were plastered with lime (gir).

“Lime is compatible with Maltese stone and allows it to breathe,” said Mr Magro Conti.

Sandblasting, power washing and using a pick axe all remove the protective patina of the stone.

“Once this patina is removed, it will not form again and the stone will continue to deteriorate,” he added.

Power washing is used to remove layers of paint or the first layer of stone. Mr Magro Conti explained that a strong jet of water is blasted onto the surface and almost immediately blows away the outer surface of the stone.

“Apart from the powerful pressure, which causes micro-cracks in the stone, water is forced deep into these cracks,” he said.

Although these methods are extremely time-efficient, the stone is pitted. “If the stone is exposed to the elements, it will start crumbling soon after.”

The use of pressurised water is also damaging as it over saturates and softens the stone. Water is only used in special circumstances under supervision of professional conservation architects.

The water needs to be completely desalinated and the pressure kept very low. The application is called nebeuliser and the angle of application is controlled.

Mr Magro Conti said it is used to soften the soot and grime rather than remove it “mechanically”.

Although many people are under the wrong impression that sandblasting and power washing creates a “rustic” effect and strengthens the stone, these methods simply accelerate its deterioration.

“We do not recommend using these techniques anywhere,” said Mr Magro Conti. “It is true that it is a fast method but it causes great damage.”

Unfortunately, they are used in areas of great beauty and historical value like in Valletta and Mdina, he pointed out.

“People who take great pride in keeping their house immaculate are in the greatest danger as they believe that by using sandblasting and power washing they are restoring their house to its former glory,” he said.

“In the last three years, I noted a great change in Cospicua – there are more houses on which sandblasting was used,” said Mr Magro Conti. “More people are buying property there and ‘restoring’ them.”

Instead of using these harmful methods, he recommends taking an expert’s advice before taking any action.

“There are several architects specialised in this sector. You can’t say that one method will solve certain problems as each case is individual and needs individual treatment.”

Although the recommended treatment might be more expensive and more time-consuming, it will restore the property to its former glory without ruining it.

“These solutions are not quick-fix-it and cannot be bought at the local ironmonger. It takes patience and is slow work,” he said.

It is recommended that manual scraping is done using flat sharp hand tools and without aggressive application so the original stone surface, tool marks and patina are not damaged.

Although it is a slower process, it prevents damage to the surface and protects the internal consistency of the stone, he explained.

“Therefore the authentic fabric of each stone is safeguarded as well as the cultural and economic value of the property,” he said.