The lifetime of the coating is significantly affected by its ability to adhere to the substrate. It is generally acknowledged that correct and thorough surface preparation is one of the most important factors relating to the successful application of a coating or surface treatment. It is also critical to the effective lifetime of the coating. It is therefore essential, for any coating to perform successfully, that the substrate is prepared properly.
The level of surface contaminants such as oil, grease, dirt, existing coatings, corrosion, salts, etc. which are present on the surface prior to coating, needs to be carefully assessed and an acceptable contamination level agreed by all parties. Some standards that define levels of contamination are referenced in specifications. The main factors affecting the performance of a coating system, primarily on steel, can be split into four areas:
Surface Condition: degree/ percentage of rust, level of mill scale, etc.
Visible Surface Contamination: Oil, grease, dust and amine blush on amine cured epoxy coatings.
Non-Visible Surface Contaminants: Surface salt and ion specific contamination (sulphates, chlorides, nitrate).
Surface Profile: In particular the peak-to-valley height and/or average roughness of the profile.
The overriding aim of surface preparation is to remove all such surface contamination and, in the case of protective coatings, to create an acceptable level of surface profile such that:
1: Sufficient coating adhesion can be obtained
2: The risk of corrosion is minimised
3: The coating achieves its service life parameters
Creating a surface profile increases the surface area of the substrate, which will improve the adhesion of the coating. A profile that is higher than specified will require more of the coating to be applied so that the peaks of the substrate are sufficiently covered to avoid rust spots. This subject is covered in more detail later in this section.
When coating any substrate, the first step is to assess the condition of the surface.
Once the surface profile meets the specification, the surface should be assessed for cleanliness.
Important parameters need to be controlled/monitored during blasting.
The surface appearance of the steel after blast cleaning is referred to in the pictorial standards.
Once the surface profile is at the required standard then the surface cleanliness should be addressed.