Recently, in the building industry, attention has been directed to use, as a finishing material, applied concrete itself, pre-casting concrete, a hollow extrusion-molded concrete plate and the like.
However, such material has serious problems in that it is easily turned to a darker color by rain water and stained by air pollutants, thereby spoiling the original good appearance.
To cope with the same, various attempts have been made to produce an effective, clear protective coating.
However, in such method of coating a concrete surface several times with a clear coating composition comprising a resinous varnish obtained by diluting a synthetic resin with an organic solvent and combined with various additives as a defoaming agent and the like, one cannot attain the desired object of obviating the darker color because the concrete color is darkened by the applied coating composition. Moreover, there arises a new question of turning to a different material as a finish because the applied coating composition has the effect of strengthening the shade of color of the concrete having a heterogeneous composition.
Even if there is used a slightly colored clear coating composition added with a small amount of coloring pigment to offset the darkened color in place of the clear coating composition, it does not completely cover the substrate material, and there still results the problem of darker color; hence, one is unable to make the most of the original feel and appearance of the concrete.
When a water diluted, synthetic resin emulsion composition is applied by a coating technique on a concrete surface, differing from the above-mentioned solvent type coating composition, to alleviate the darker color problem, such composition is comparatively poor in adhesion to the concrete surface, often resulting in blistering and peeling of the applied coat. This is due to the action of rain water and the water contained in said concrete.
Attempts have also been made to coat or to impregnate concrete with a penetrable water repellent as an alkyl alkoxy silane and the like. However, such material is gradually dissolved out of and removed from the concrete and its activity can be only expected for at most 2 years.
It has also been proposed to first coat or impregnate the concrete with a water repellent, such as alkyl alkoxy silane and the like and then apply a solvent type clear coating composition thereupon; however, in this case, there is a problem of the generation of a darker color.
Thus, none of the proposed methods have solved the above-mentioned problems.
It is, therefore, an object of the invention to provide a method for preparing a clear coat on a concrete surface, without causing any loss in the quality and material feeling of the concrete and capable of effectively preventing the generation of this darker color and which is excellent in adhesion and durability.