Surface Preparation and Painting
In protecting steel with paint, careful surface preparation is of utmost importance. If this surface preparation is neglected, the performance of the paint cannot be ensured.
(1) Surface Preparation of Bare Steel (Pre-Fabrication Treatment)
- Remove oil and grease by solvent cleaning after scraping away firmly adhering impurities.
- Corrosive salts and any sulphates on steel surface are to be removed with fresh-water washing. The surface is then to be dried with dry waste cloth or dry compressed air.
- All mill scale, rust and foreign matter is to be removed by shot-, grit- or sand-blasting. Finally, the surface should be cleaned by blowing down with compressed air.
Treated bare steel surfaces rust rapidly when exposed to the air, and therefore should be painted with a shop primer as soon as possible in order to prevent re-rusting during storage, fabrication and fitting out. The main function of shop primers is to provide steel with temporary protection against rusting during fabrication and fitting out. Consequently, rapid drying so as to permit handling in a short time after application and welding or gascutting properties are required for the shop primer.
Shop primers must also be compatible with any subsequent paint.
(2) Surface Preparation of Shop Primed Surface (Secondary Surface Preparation or Post-Fabrication Treatment)
Water and moisture, oil and grease, white rust, chalk marks, unsuitable marking paint and other contaminants should be removed by solvent cleaning and/or suitable degreasing. If the shop primed surface has been damaged during rolling, cutting or welding, or suffered mechanical abrasion in storage, handling and transport, the affected areas should be cleaned by wire brush or disc sander and touched-up. The entire surface must be cleaned by washing, if necessary, before subsequent paints are applied.
(3) Surface preparation for Old Paint Film
The surface must be cleaned by scraping and/or solvent wiping and/or fresh water washing to remove salt, dirt, oil and grease and other impurities. All rust, oil, loose paint film and other impurities should be removed by disc sander, wire brush or other suitable method after which a specified paint system should be applied.
(4) Dry Abrasive Blast Cleaning
These guidance notes are intended to be a tool for the visual assessment of preparation grades by means of dry abrasive blast cleaning.
The surface finish achieved by means of dry abrasive cleaning depends on the original surface condition as well as the type of abrasive blasting equipment, size, hardness, type and abrasive shape.
The original surface conditions of steel may be one of four of the following rust grades:
A Steel surface largely covered with adhering mill scale, but little, if any rust.
B Steel surface which is beginning to rust and the mill scale is starting to flake.
C Steel surface on which the mill scale has rusted away or from which it can be scraped and with slight pitting visible under normal vision.
D Steel surface on which the mill scale has rusted away and on which general pitting is visible under normal vision.
(5) Preparation grades for dry abrasive blast cleaning.
Surface cleanliness is divided into four grades, designated by the letters ‘Sa’.
Sa 1: Light blast cleaning or brush-off.
Sa 2: Thorough blast cleaning or commercial blast.
Sa 2.5: Very thorough blast cleaning or near white metal.
Sa 3: Blast cleaning to visually clean steel or white metal.
The most ideal preparation is blast cleaning, however this may not always be possible due to environment, prevailing factors, cost issues, etc. Although coatings and coating systems have been prepared, care should be taken with the type of mechanical pre-treatment methods use.
(6) Preparation grades for Mechanical Cleaning:
St 2: Thorough hand and power tool cleaning
St 3: Very thorough hand and power tool cleaning
Types of Mechanical Cleaning:
- Hand Scraping
- Hand Brushing
- Wire Brushing
- Needle chipping
- Chisel Hammering
The results produced from the above mechanical cleaning may result in poor surface preparation and the danger of polishing or chipping the surface area.
Full advantage should be taken of weather conditions to carry out painting when the weather is favourable. Paint should never be applied on to a wet surface. Not only should painting be avoided in rain, sleet or fog, but attention must be paid to the presence of condensation on the surface.
Generally, painting should be done at over 5oC and below 85% Relative Humidity (R.H.). Painting should not be carried out when the surface temperature is less than 3oC above the dew point, no matter what the R.H. is at the time.
NB. Observe instructions for application of epoxy coatings, epoxy tank coatings or inorganic zinc coatings.
(2) Preparation and Storage
(a) Some paint components, although perfectly stable at normal temperature, will react together at higher temperatures, causing thickening, etc. Some paints such as water based paint should not be stored where temperatures fluctuate excessively.
(b) Shelf life If stored in normal conditions
… 18 months maximum, subject to re-inspection thereafter
Modified Acrylic, Epoxy
… 12 months maximum, subject to re-inspection thereafter
… 6 months maximum, subject to re-inspection thereafter
(c) When paints are left standing they very often tend to separate slightly. This is caused by the different specific gravities of the ingredients. All paint must, therefore, be carefully stirred and mixed until homogenized before using. Stirring should be done from time to time in order to prevent such separation, while painting.
(d) Once opened, some paints rapidly form a skin on the surface. Generally the quick-drying types have this tendency, this being no fault of the paint, but is due to the natural process of drying. The skin must be removed before painting. This precaution should be applied particularly as the skin may clog the spray equipment.
(e) Most paints do not normally require adjustment but under conditions of excessively high or low temperature a small amount of appropriate thinner, not exceeding the amount specified by the manufacturer, may be added in order to ease brushing or to bring paint to spraying consistency.
(3) Film Thickness
The dry film thickness of shop primer on a blasted steel surface should be measured as follows: Place a smooth steel panel on blasted steel surface, apply shop primer and measure the dry film thickness on the steel panel with an adequate electromagnetic thickness meter.
(4) Application Method
The paint should be applied on the surface by lengthwise and crosswise movements of the brush. Rough surfaces, rivet heads, edges and angles should be given special attention with the application of a stripe coat.
(b) Paint Roller
Cover the surface of the roller by spreading the paint. Paint at a slow and even pace up, down and across. Do not spread the paint excessively. Particular care should be taken when painting rivet heads and welding seams.
(c) Airless Spray
Most products can be applied by airless spray, which is still more effective and economic method than conventional spray, especially on large areas. The method is quicker with less spillage and the paint can be applied in thicker coats. The use of airless spray requires more of a routine than conventional spray and great caution must be exercised in handling the spray gun, which works with very high pressure.
(5) Cleaning of Tools
The tools should be cleaned immediately after use. This is particularly important when working with quick-drying or two component paints. Spray equipment should be cleaned after use by flushing with an adequate thinner. Special care should be taken in cleaning the nozzles.
(6) General Safety and Health Protection
Refer to the Safety and Health Protection page in this manual for all matters pertaining to the protection of skin and eyes as well as the dangers of ingestion and inhalation when using our paint products irrespective of the method of application.