White walls yellow, black how to do?

White walls dirty can scrape white, scraping white is now a means of decorating the interior walls of the building, the thickness is generally in 1mm or so, after drying the wall is very white, the effect is very good, generally will not be easy to fall out of color. First of all, use sandpaper to polish the wall, in the construction of the special use of white powder, or talcum powder, cellulose synthetic mixture of the wall to scrape and brush white. You can also spray yourself with white self-spray paint, usually two times.