When applying overglazes it is critical that both the object and work area be clean and dry.  Any contamination by dust or moisture will interfere with the overglaze application resulting in defects.  Clean the glaze surface before application with alcohol and allow to dry completely.  Use a clean, dry, soft bristled brush.  Saturate the bristles with overglaze and work out the excess.  Use firm, even strokes one next to the other.  Apply as evenly as possible.  Allow to dry before firing.

During firing, kiln should be stacked loosely and well ventilated.  

The following applies to oil-based Overglazes.  

There should be no moisture in the brush or you can have bare spots in the fired finish.  For this reason, you should not clean your brushes in alcohol which contains water.  Instead use a brush cleaner.  It is best to keep one brush per color to be used just for that color.  Clean brushes immediately after use.  

Overglazes can be thickened by placing a small amount on a glazes surface and allowing some of the medium to evaporate.  Thickened overglaze can be re-thinned with Hanovia Essence or Gold Essence.

When using both a metallic and a luster color on the same piece, it is best to avoid contamination by firing them one at at time.  Fire the metallic first, then the color.