Surface Preparation and Cleaning. Table of content
Section |
Title |
Page |
6.1 |
Introduction |
6.2 |
6.2 |
Pre-treatment of steel surfaces |
6.2 |
6.3 |
Steelwork (Pre-blasting preparation) |
6.3 |
6.4 |
Cleaning |
6.4 |
6.5 6.5.1 6.5.2 |
Pre-treatment Hand and power tool cleaning Blast-cleaning |
6.5 6.5 6.7 |
6.6 6.6.1 6.6.2 |
Abrasives Metallic abrasives Non-metallic abrasives (Mineral abrasives, natural and synthetic |
6.10 6.13 6.14 |
6.7 |
Water jetting and hydroblasting |
6.15 |
6.8 |
Electrolytic descaling with Magnesium strips |
6.16 |
6.9 |
Preparation between paint coats |
6.20 |
5.1.1.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.
Common tools for hand tool cleaning are sandpaper, non-woven abrasive pads, wire brushes, chipping hammers, scrapers etc. 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.
6.5.1.2.Power tool cleaning.
There are two main types of power tools; the rotary cleaning tools and the 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 millscale, rust and weld slag, but tend to smoothen or burnish a surface and give bad adhesion for a coating.
Coated abrasive (disc grinding) can remove paint, loose millscale and rust, but tends to clog up when paints 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.
Common impact tools are:
Pneumatically driven hammers or rotary hammers.
Needle guns
Chisels.
These impact tools are very noisy, slow and subject to damaging the steel substrate, but are effective for removals of rough scale, including heavy laminated scale. The use of needle guns should be limited to welds, corners, uneven edges etc. as the impact of the needles can cause an unacceptable profile on flat surfaces. “Non spark” tools may be necessary in areas subject to fire or explosion hazards.
ISO 8501 have two grades of cleanliness after hand and power tool cleaning on rust grades B, C and D: St. 2 and St. 3.