4 Ways To Successfully Prepare Your Surface Ready For Painting

Author Andy Potts | Reading time approx 2 mins

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.   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 8501-1 has two grades of cleanliness after hand and power tool cleaning on rust grades B, C and D St 2 & St 3. 1 → Needle Gun , 2 → Pneumatic Chipping Hammer Abrasive Blast Cleaning - Sandblasting Blast-cleaning is the common term for all methods using abrasives that are propelled toward the surface to be cleaned at very high speed. Different kinds of abrasives are propelled either by high velocity compressed air, centrifugal force or by a high-velocity stream of water to remove rust, mill scale, existing coatings and other contaminants from the substrate.    The different methods of blast-cleaning are :  

• Centrifugal abrasive blast-cleaning
 
• Compressed-air abrasive blast-cleaning
 
• Vacuum or suction-head abrasive blast-cleaning
 
• Moisture-injection abrasive blast-cleaning
 
• Wet abrasive blast-cleaning
 
• High pressure and ultra high pressure water-jetting

Of the above, the most common methods today are dry abrasive blast cleaning, centrifugal abrasive blast cleaning and high pressure water jetting.

Centrifugal Blast Cleaning

Centrifugal abrasive blast-cleaning is carried out in fixed installation or mobile units in which the abrasive is fed to rotating wheels or impellers positioned to throw the abrasive evenly and at high velocity on to the surface to be cleaned. The abrasives used are usually a mixture of steel grit & shot, and the abrasives are re-circulated.

This method is suitable for continuous treatment of plates for removal of the mill scale & surface corrosion and for profiling the steel plate. Normally, in connection with the centrifugal blasting machine there is an application unit which automatically applies a shopprimer or prefabrication primer on the steel. Major yards have their own plant, while smaller yards buy their steel ready primed from specialist companies or steel manufacturers. 

The blast-cleaning standard should be Sa 2 ½ (according to ISO 8501-1) before the shopprimer is applied. The shop-primer’s paint inspector should check the plant and ensure that this degree of cleanliness is maintained before the shop-primer is applied.  However nowadays, the major part of construction steel is delivered centrifugally blast cleaned & shopprimed, allowing little opportunity to check the quality of the shot blasting.

The shop primer, which is intended to provide temporary protection, is applied to a specified film thickness by an automated process. The thickness is usually 15 - 20 microns for iron oxide shopprimer and 10 - 15 microns for zinc shopprimer. It is very important to keep inside the specification, because excessive shopprimer thickness will reduce the cutting and welding speed and thereby prolong the production time at the yard.

 

Centrifugal Blasting - Shop Priming Plant - Video Courtesy of Jotun Paints

 

Pickling

Acid pickling is an old established shop method of removing millscale from steel. There are several detailed processes still in use, often employing a duplex system of aggressive and passivating acids. One disadvantage of pickling is that it leaves the steel clean but without the anchor pattern, which is an aid to adhesion with high, build heavy-duty coatings.

 

Flame Cleaning

Flame cleaning processes are less frequently used today. In common with other low standard surface preparations like handtool cleaning or low-grade blast cleaning, they do not remove all adherent mill scale or rust and therefore, are not suitable for sophisticated heavy-duty coatings. Combustion by products may contaminate the surface. The main advantage of flame cleaning is that it removes some chemical contamination and leaves the surface warm and dry, which aids the drying of the initial priming coat and assists good adhesion.

Flame cleaning of anything other than small surface areas is now restricted in many parts of the country as this method of cleaning is seen as contributing to air pollution.

Solvent Cleaning 

This is not a true surface preparation, but rather is used to remove oil, grease and similar contaminants via the action of solvents or emulsifiers. 

Apart from the hazards of flammability and sensitisation to the skin, care should be exercised when using solvents to use clean rags and to change the rag often so that dissolved hydrocarbon contamination is not spread over a wider area as a thin, invisible contamination layer that will later interfere with coating adhesion. Solvent resistant gloves and eye protection shall be worn. Never use solvents in a confined space without an appropriate respirator or air supply.

Solvent cleaning alone does not fully remove oily contamination and should only be used as part of a process involving washing with water miscible cleaning agents, detergent, surfactants or oil emulsifiers.

Many coating specifications now restrict the use of solvents to only being used to remove identification crayon and isolated oil & grease deposits. Larger surface areas should be washed with a biodegradable alkaline cleaner / degreaser.

 

For further information on sandblasting, degreasing, primer paint or top coat paint in indsutrial scenarios please leave a comment below.