The Key to a Good Coating Job Mechanical adhesion of a paint is associated with surface roughness or “anchor pattern”. Anchor pattern is the surface profile formed by peaks and valleys on the substrate. By increasing the number of hills & valleys,
the surface area of the substrate increases which in turn gives an improved adhesion. Blast-cleaning to a surface roughness of 50 microns may increase the surface area up to 45-50 % compared with a smooth plate. Surface preparation methods include both mechanical cleaning and blast-cleaning. For mechanical cleaning we have two main methods, hand tool cleaning and power tool cleaning. For blast-cleaning we have several methods which will be described later in this chapter. For large surfaces, mechanical cleaning is more expensive than blast-cleaning. It is therefore used mainly for local repairs, removal of rust spots and treatment of damaged or burnt areas and welding seams. Results can be very variable and the process will generally be a relatively slow one in order to be effective. On heavily rusted surfaces, heavy deposits have to be removed before more thorough surface preparation methods are employed. After hand or mechanical tool cleaning and prior to painting, re-clean the surface if it does not conform to the specification. Prior to painting, remove dirt, dust, or similar contaminants from the surface. Acceptable methods include brushing, blow off with clean dry air, or vacuum-cleaning. Hand Tool Cleaning Hand tool cleaning is one of the oldest methods of surface preparation of steel surfaces. A hand tool cleaned surface is free from loose rust, loose paint and loose millscale. It is not intended that adherent millscale, rust and paint should be removed by this process. Millscale, rust and paint are considered adherent if they cannot be removed by lifting with a dull putty knife. Scraping, wire brushing and chipping are used as methods for removing rust & old paint. Before hand-tool cleaning, make sure that dirt, grease and oil have been removed from the surface. A hand tool cleaned surface is desirable for applications where a low cost cleaning method is required and a short life paint system can be tolerated. A coating with good penetrating properties (epoxy mastic) gives the best result. When scraping or wire brushing manually, much effort is needed to remove as much rust as possible. Since only a few square feet per hour can be satisfactorily cleaned, this method should be limited to small areas and when a high level of surface cleanliness is not required. Power Tool Cleaning There are two main types of power tools;
- Rotary cleaning tools
- Impact tools
In general the rotary cleaning tools are the most effective ones in removing paint and rust, while the impact tools are more effective in removing scale and pack rust. Common rotary cleaning tools are wire brushes, coated abrasive and non-woven abrasive. Wire brushes can remove paint, loose mill scale, rust & weld slag, but tend to smoothen or burnish a surface and give bad adhesion for a coating. Do not use high speed wire brushing as a surface preparation method before painting. Coated abrasive (disc grinding) can remove paint, loose mill scale and rust, but tends to clog up when paint is removed and may remove some of the substrate material. Non-woven abrasive does not clog up as much when removing paint and it can take a surface to bare metal and yet remove only negligible bare metal. full house cleaning services