It is now widely understood that correct surface preparation is the most important single factor in controlling the success or failure of a painting operation. Even the best paints or coatings may fail on a badly prepared surface, whilst the simplest and cheapest paint may perform surprisingly well on a correctly prepared surface. All paint coatings, without exception, allow the passage of some water and oxygen through the film. Amounts vary according to the thickness, natural permeability and integrity of the applied film, but in all cases it is reckoned that sufficient water and oxygen can pass through a paint film to rust steel at the same rate as if the paint were not there. In theory therefore, paints protect not by exclusion but by putting a high resistance path (i.e. impeding the movement of ions) in any potential corrosion cells. In practice, paint films very often contain micropores or pinholes which allow an even larger transport (relatively speaking) of water through the film, but the effect of this is considerably lessened if it cannot creep at the interface. Good adhesion is therefore on of the biggest single factors in paint protection, and the biggest single factor in good adhesion is good surface preparation.