Why do fillers sometimes cause changes in the colour of the paint finish?

1. Fillers are absorbent and have a different porosity than the substrate, which can cause variations in colour. Therefore, fillers should be applied as early as possible in the process to avoid variations in gloss, sheen or colour.
2. All purpose fillers contian cement and have a high alkali content. Interior grade fillers are based on plaster rather than cement and do not cause changes in colour. To remedy the situation, prime the affected areas with two coats of Speed Undercoat prior to repainting. If the problem is extensive you may need to prime the whole surface to ensure an even finish.