Rejection, commonly known as the Silicone Effect is a pullback of a coating applied to a substrate. With Juju this is often manifest as a snowflake like dandruff as the coating delaminates. Rejection
is caused by the presence of contaminants, either in patches, or all over the floor surface.
The most common contaminants which can cause a timber floor finish to fail are:
- Timber extraction (resin, wax, oil)
- Aerosol sprays
- Food spillage
- Silicone contaminated equipment
- Strong solvents getting under existing coatings
It can also be a result of disparate materials having a negative interaction, as in the case of some 2 packs.
Unfortunately, in most cases it is not possible to predict rejection on a floor prior to the coating application. Signs of rejection normally occur
after application of the first or second coat during the initial drying process and evaporation of the solvents. A contaminant may be picked up
in an isolated spot on the floor during the sanding process and could be spread all over the timber or cork surface.
Another problem can occur when the strong solvent xylene used in Juju is able to get under the edge giving a “fried egg effect”.
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 otherwise the coating cannot wet and pulls back from the surface.
Toby Flow Add is a specially designed additive
to reduce the surface tension of all Toby Polyurethane cork and timber finishes. We have also had success with Urethane Coatings Flowmax. Generally an addition of less than 0.5% (5ml per litre) is sufficient
to overcome rejection problems. The best thing of course is to start with the recommended amount and work up from there with small test pieces.
When additives are used in a coating, the next applied coating should also contain at least the same quantity of because of the reduced surface tension.
Always properly clean old floors in order to remove contaminants
including grease, oil, wax etc. before the sanding process is started. Toby Polyclean in warm water or Toby Reducer Thinners on problem floors
will do the job in most cases, as will white spirit. Plenty of clean rags should be used when cleaning the floor to ensure that contaminants are
removed and not spread all over the surface. Care should be taken with disposal of these rags as they remain highly flammable.
It is better to throw away equipment like sandpaper, screen backs, or rollers if they were used on a problem floor as contaminants may be transferred
to following jobs.
Silicones can be a serious problem, therefore always check if such
materials were used on or near the floor. The removal of contaminants from the floor prior to the coating application is critically important.
Even when a floor has been cleaned and is free of rejection, a high concentration of certain contaminants remaining on the floor may weaken the
bond between the coats and later cause delamination.
Feel welcome to call and discuss further with one of our friendly sales staff.