It is very essential to examine the water –bound macadam carefully before deciding to surface dress it. (See Patching of Potholes under preparation of base for Laying Bituminous Courses.) If the existing surface shows any depressions, potholes, ruts or irregularities exceeding 12mm or has a steeper camber than 1 in 60, it must be re-conditioned. If the surface is not so bad as to warrant resectioning it should be patch repaired a week or two before the surface dressing is to be started, and left open to traffic. Stone metal of the old untreated road surface should be exposed to a depth of 6mm to 12mm to give the binder a better opportunity of penetrating and binding the road. Care should, however, be taken to see that the stability of the road is not disturbed and the stone metal is not loosened.
It is most important that the surface of the road shall be thoroughly cleaned before the application of a binder. It shall be swept clean and free from dust , dirt, caked earth and any other extraneous matter. The cleaning shall be done first with hard wire brushes, then with softer brushes or base brooms and finally by fanning or blowing with gunny bags or sacks to remove all lose dust. The road must be absolutely dry before painting is commenced and this dry condition must extend right down to the earth subgrade or for at least 6cm incase of emergency.