Fixing > Substrate Preperation - for Glass Mosaic

Correct substrate preparation is of basic importance for a perfect end result, both technically and visually.

1. How to Prepare Substrate       

• Only use cement-based plaster when fixing Palladio products. Even with plaster it is essential that the right curing times are respected as well as the correct planarity. The covering on a well-prepared surface gives an overall depth, adhesive plus mosaic, of approx. 6 mm.

• The first thing to ensure is that the surface to be covered is smooth, dry and flat i.e. not out of plumb. In case of specific requirement, the surface should be made waterproof.

2. Product Specific Requirement 

• If the surfaces are shaped, they cannot have bends with less than 6 cm radius when fixing 2x2 cm mosaics and 3 cm for the installation of 1x1 cm mosaics.

3. Physical Diagnosis       

• It is very important that there is no rising damp or salt efflorescence on the surface of the substrate, in which case fixing will only be possible after having eliminated all the causes of the problem and removing any efflorescence.
• Before beginning the fixing, remove all traces of dirt, oil, grease, wax, cement grout and, if removing resilient coverings, adhesive residues.
• In the specific case of newly built swimming pools, a leaking test has to be made to assess the structure's behaviour after settlement as well as the efficacy of the waterproofing of the pool. This test consists in filling up the pool, after completion of the drying period and waiting for a minimum of two weeks, after which there should not be a significant loss in the volume of water.
• Whether the screed is a newly laid one or an existing one, it must be perfectly clean, free of objects, without shrinkage cracks or surface defects before fixing the flooring. Moreover it must have the right consistency and should not crumble.
• If fixing on existing material, the old wall cladding must be well bonded & in level, so as to avoid detachment.

4. Technical Diagnosis       

• The substrate must be compact and free of cracks. If it tends to crumble or it shows signs of shrinkage or cracks it is not suitable as a base for fixing.
• In case of new substrate, it is important to know the curing time and level of residual moisture. In case of cement screed ideal curing time is 28 days and maximum residual moisture is 5%. While in case of concrete, ideal curing time is 3 months and maximum residual moisture is 3%. Please bear in mind that both parameters must be respected.
• If the structure requires expansion joints, they should be planned in the building project and completed before fixing begins.

5. Corrective Action

• Surface-cracked substrates: Network of tiny surface cracks are no major cause for concern, since these cracks are merely superficial and inactive. In other words, they are not and will not become subject to movement. No prevention action is required with such conditions.

• Fractured substrates: These deep cracks or fractures are more apparent and may range in size in width. They affect the majority of the substrate thickness and are liable to move with seasonal climatic changes or changing humidity levels. If the fractures are no more than a tenth of a millimeter, glass fibre mat can be embedded in the first coat. Larger fractures must be sealed definitively with appropriate adhesive.

• Crumbling substrates: To determine whether or not the surface is crumbling it is sufficient to draw a nail across the surface. The surface should not crumble to any extent under this pressure, and any incision left by the nail should be sharp. With some substrates, merely rubbing a hand over the surface creates a fine dust which, even if limited to the surface layer, is detrimental in case of smoothing. In this case too, intervention and correction is fundamental with proper screed / adhesive.

• Structural cracks, hollow surfaces, scaffolding holes and gaps must be rectified before fixing by proper filling with rich concrete mortar. In certain specific cases, chicken wire mesh is to be used for surface preparation instead of commonly used V - cut method.