It’s entirely possible to cover over a surface that’s not been prepared properly first, but the results will not last the test of time. Washing prior to painting is a crucial part of the process, this makes sure the paint surface is free of dust, debris, mould and mildew, all of which if left on the surface, would greatly impact the paints adhesion to the trim, walls and even ceilings.
Get the job done professionally, see our house painting services page for more detail.