BGAS Painting Study Material Chapter 2

SURFACE PREPARATION METHODS & STANDARDS

If paint is applied over the corrosion reactions, and other contaminants,  

1. The poor adhesion of the coating and thus the coatings life would be far from satisfactory. 

2. A good surface preparation grade (degree of cleanliness) along with a suitable surface profile can give 10 years life from a typical four-coat paint system.  The same system applied over a substrate with little or no profile and contaminant remaining might give four to six years, or even less.

Surface Preparation

Involves removing these contaminants, and in some instances increasing the area available for adhesion by roughening up the substrate.

Therefore two factors need to be considered when inspecting a surface preparation.

1.         Degree of cleanliness

2.        Surface Profile (degree of roughness)

Surfaces can be prepared for paint application in several different ways; each one varies in cost, efficiency, ease and suitability.

a)      Dry Abrasive Blast Cleaning

b)      Water Blasting

c)      Hand and Power Tool Cleaning

d)      Flame Cleaning

e)      Pickling

f)      Vapour Degreasing

g)      Weathering

Dry abrasive blast cleaning

A. Dry abrasive blast cleaning involves compressing air and forcing it along a hose and out of a small aperture (gap) called a nozzle.

B. A pressure of 100 psi results in the air speed exiting the nozzle at approximately 450 mph.

C. Abrasive particles are mixed in with the air and travel at the same speed; they will carry a lot of work energy.  This energy is used in chipping away mill scale and other detritus from the substrate.  And shattering into small pieces and with others all the energy is used in impinging into the steel surface, roughening the surface and increasing the surface area to increase adhesion properties.

Because all standards refer to the amount of contamination remaining on the surface, (The longer the time spent on this operation, the higher the degree of cleanliness.)

Abrasives

Abrasives come in many forms and can be classified in several different ways, as shown below.

None metallic (Mineral) Expendable	Metallic (Recyclable)	Agricultural by-product
Copper Slag  Nickel Slag  Boiler Slag  Glass Bead  Aquamarine  Garnet  Sand	ACI (Angular Chilled Iron) Steel Grit Steel Shot Grit and Shot Mix Garnet	Walnut Shell Coconut Shell Eggshell Corn Cob Husk Peach Husk

continue......

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