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Diagnosis and preparation of the wall
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The solidity of the fresco and its duration depend mainly on the state of the wall. It is essential to observe it well and to make it healthy, robust and dry.
- The cracks should be repaired and the holes stopped.
- An old coating which " sounds hollow " must be removed.
- Any trace of moisture (aureoles, moulds, punctures, chalking) must be determined and its causes treated.
- The wall should not contain any hidden source of humidity. If there is a doubt, only a specialist will be able to indicate the proper technique of insulation.
- Any trace of painting must be scraped, striped with the peak or the bush-hammer in order to allow a good adherence to the mortar.
- Any trace of plaster should be eliminated. Diluted solutions of hydrochloric acid can be used but with special care.
- For an outside fresco, study the exposure to the sun and the rain of the selected site. The best protection is eaves of roof or a porch roof. The exposure of the site should be well known before working.
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Degradation of an old painting layer with the bush-hammer. |
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The gobetis
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The wall should have a rough surface to which the first coat can adhere. For this purpose, smooth surfaces - as concrete in particular - must be prepared by a gobetis. If there is any doubt about the mortar adhering to the support (wall), this is a solution.
The gobetis consists in diluting pure Portland cement in water and projecting this mixture on the wall with a stopper or using a small machine( called " tyrolienne ") to project the liquid mortar onto the wall. Once dry, the gobetis has a sufficiently rough surface to make the mortar adhere. |
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Application of a gobetis on a concrete surface.
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Gobetis on a friable brick wall. .
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