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. All purpose fillers contain cement and have a high alkali content. Interior grade fillers are based on plaster rather than cement and do not cause changes in colour. If the problem is extensive you may need to prime the whole surface to ensure an even finish.