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Surface Preparation

The objective of applying a paint coating is provide a film that will protect and decorate the surface being painted.
How any paint coating performs, depends on the correct and thorough preparation of its surface. Given below are some of the recommended surface preparation techniques commonly followed :

Degreasing
First remove all traces of oil, drilling / cutting compounds and any other surface contaminates. The most common method is by solvent washing, followed by wiping dry with clean rags. Recommended procedures are described in SSPCSP-1.

Hand Tool Cleaning
Loosely adhering millscale, rust and old paint coating may be removed from steel by chipping, scraping, hand wire brushing and emery paper cleaning. However, these methods are incomplete, and may leave a layer of tightly adhering rust on the steel surface. Procedures for hand tool cleaning are described in SSPC-SP2.

Power Tool Cleaning
For the removal of loosely adhering millscale, paint and rust, this is more effective and less laborious than hand tool cleaning, it will not however remove tightly adhering rust and millscale. Power wire brushes, impact tools such as needle guns, grinders and sanders may also be used. Care should be taken, particularly when using power wire brushes, not to polish the metal surface, as this will reduce the key for subsequent paint coating.

Methods are described in SSPC-SP3 and should be to Swedish Standard ST 3.

Flame Cleaning
Slightly more effective than power tool cleaning, but has the inherent risks of fire and explosion, and of possible distortion of light gauge steel.Blast Cleaning By far the most effective method for removal of millscale, rust and old coating by using abrasives such as sand, grit or shots under high pressure. Prior to blasting, steelwork should be degreased and all weld spatter removed. Weld seams and sharp edges should be ground down. This is because paint coating tend to run away from sharp edges, resulting in thin coatings and reduced protection.

The grade of blasting suitable for a particular coating specification depends on a number of factors, the most important of which is the type of coating system selected. There are four commonly used grades of blast cleaning, and the approximate equivalents between the various Interantional Standards are as follows :

 

 

USA Spec. SSPC

NACE

British Std. BS 4232

Swedish SIS 05 5900

Japanese Std. SPSS

White Metal

SSPC-SP5

NACE 1

1 st quality

SA 3

 

Near White Metal

SSPC-SP 10

NACE 2

2 nd quality

SA21/2

JA Sh2 or

JA Sd2

Commercial Blast

SSPC-SP 6

NACE 3

3 rd quality

SA 2

JA Sh1 or

JA Sd1

Brush Off Blast

SSPC-SP7

NACE4

-

SA 1

-

The profile of roughness obtained during blasting is important, and will depend on the abrasive used, the air pressure and the technique of blasting. Too low a profile may not provide a sufficient key for a coating, while too high a profile may result in uneven coverage of high sharp peaks leading to premature coating failure, particularly for thin coatings such as blast primers.

The following table gives a brief guide to typical roughness profiles painted using various types of abrasives :-

 

Type of Abrasive

Mesh Size

Max. Height of Profile

Very fine sand

80

37 microns

Coarse sand

12

70 "

Iron Shot S 390

14

90 "

Typical non-metallic "Copper Slag"

1.5 - 2 mm grain size

-

75 / 100 "

Iron Grit No. G 16

12

200 "

The accepted methods of applying the protective described in this manual are by brush, roller, conventional spray and airless spray. The advantages and disadvantages of each method are briefly discussed below.

Brush Application
Brush application is a relatively slow procedure, but is generally used for decorative paints and for coating small areas. It is particularly suitable for coating complex areas where the use of spray methods would lead to considerable losses due to overspray and associated dry spray problems.