Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2005

Colour (Color) Matching and Colour Differences with Touchup

Q. We are a building contractor in Hong Kong. We had recently completed a facade renovation work for a building. There was repainting work to the sprayed textured painting of the facade. We had repainted it and after more than half a year, we touched up some defective works upon the request of the project surveyor with the residual paint (SKK's "Ceratight U" 30% gloss, white colour - solvent based ceramic polyurethane resin protective top coat). However, the surveyor complained there was colour difference between the 'repainting' and the 'touch-up painting'. The surveyor requests us to repaint the whole building at our own cost. Could you please give your comment and advice for us?

A. Touch up painting can cause color (colour) differences for a variety of reasons:

1. Low gloss paints reflect light in all directions. Depending on how the paint was originally applied and how it was touched up you might easily see a colour difference. With low gloss paints and metallic pigmented paints the manner in which the pigments rise to the surface of the coating film affects the appearance. For instance, if you used a paint spray gun, such as an airless or air-assisted airless spray gun to apply the paint to the building and you used a roller or paint brush to perform the touch up, you might well see a colour difference.

2. The original paint and the touch up paint might have come from two different manufacturing batches, but you can check this by looking at the label of the original and touchup paint.

3. The color of the original paint might have shifted during outdoor exposure and the touch up paint will then do the same after a few months of exposure.

If this is a legal issue I might be able to assist you by evaluating a sample of the original paint and touch up paints. If they are one and the same, (i.e., from the same container) I can confirm that the properties of the paint are such that it is impossible or difficult to properly match the original during touchup operations. Also, I will be able to write a report on the problems of colour matching that you can submit to your customer. I will need to charge a fee to do this.

Best wishes,

Ron Joseph