According to Polaco Master Floor Sanding , rejections are caused by the presence of contaminants on timber floor surfaces.
Commonly known as the 'silicone effect', rejection is a pullback of a coating applied to a substrate. Caused by the presence of contaminants, either in patches or all over the floor surface, rejections can cause timber floor finishes to fail. Common contaminants include:
- Timber extraction (resin, wax, oil)
- Aerosol sprays
- Food spillage
- Contaminated equipment
It is not possible to predict rejection on a floor prior to the coating application in most cases. Signs of rejection normally present themselves after application of the first or second coat during the initial drying process and evaporation of the solvents.
For instance, a contaminant may be picked up in an isolated spot on the floor during the sanding process and spread all over the timber or cork surface.
The chemistry of rejection involves the surface tension difference between the coating and the substrate. The surface tension of the applied coating must be equal or lower than the surface tension of the substrate; else the coating cannot wet and pulls back from the surface.
Even when a floor has been cleaned completely and is free of rejection, a high concentration of certain contaminants remaining on the floor may weaken the bond between the floor coatings and later cause delamination.
How to avoid rejection on timber floor finishes
- Old floors must be cleaned properly to remove contaminants such as grease, oil and wax before the sanding process
- Equipment such as sandpaper, screen backs or rollers must not be re-used if they were earlier used on a problem floor as contaminants may be transferred
- Silicones can be a serious problem, therefore it is important to check beforehand if such materials were used on or near the floor
- Removal of contaminants from the floor prior to the coating application is critically important