- •8. Inspection and control. Table of content.
- •8. Inspection and control.
- •8.1. Inspection at shop priming
- •8.2. Inspection of steel work (Pre-blasting preparation)
- •8.3. Inspection of surface preparation.
- •8.3.1. Cleanliness. Solvent cleaning, sspc-sp1.
- •Table 8.1. Methods for solvent cleaning.
- •8.3.2. The evaluation of present condition (rust grades and visual cleanliness).
- •8.3.3 Inspection of surface preparation
- •8.3.3.1. Uncoated steel
- •8.3.3.2. Previously coated steel.
- •8.3.3.3. Roughness (steel surface profile)
- •Table 8.2. Nominal value of surface profiles.
- •8.3.3.4. Abrasives for blast cleaning.
- •8.3.3.5. Water jetting
- •8.3.4. Cleanliness after surface preparation.
- •8.3.4.1. Detection of ferrous salts.
- •Figure 8.3. Detection of Ferrous salts. “Merckoquant test”: iso 8502-1, Part 1.
- •8.3.4.2. Detection of chlorides
- •8.3.4.3. Detection of all water-soluble salts.
- •Figure 8 4. The chloride concentration on a substrate may vary significantly with exposure condition and with method used for cleaning the surface.
- •8.3.4.4. Dust.
- •Figure 8.5. Assessment of dust on steel surfaces prepared for painting. Iso 8502-3
- •8.3.4.5. Welding fumes
- •8.3.4.6. Climatic conditions, iso 8502-4
- •Table 8.3. Summary of pre-treatment and cleanliness control. Checking of all surfaces
- •8.4. Inspection before and during application.
- •8.4.1. Technical Data Sheet, tds
- •8.4.2. Correct mixing and thinning
- •8.4.3. Wet film thickness (wft)
- •Climatic conditions.
- •Table 8.4. Application control: Inspection before and during the paintwork
- •8.5. Inspection after application
- •8.5.1. Dry film thickness (dft)
- •8.5.1.2. Destructive test methods.
- •8.5.1.3. Calculation of Dry-film Thickness (dft)
- •8.5.2. Curing / drying.
- •Figure 8.6. Curing test of Zinc ethyl silicate with mek. Astm. D4752-87.
- •8.5.3. Adhesion control by “Pull-off test”.
- •8.5.3.3. Cross-cut test.
- •Figure 8.7. Classification of cross-cut test.
- •8.5.4. Holiday detector
- •Table 8.5.
- •Inspection after application.
- •Table 8.5. (cont.)
- •Inspection after application.
- •8.6. Records of work and working conditions.
- •Surface preparation.
- •Cleanliness after surface preparation.
- •Mixing and thinning
8.5.1.2. Destructive test methods.
The dial gauge.
On a small area the paint film is scraped off down to the substrate. The difference in height is then measured between the area from which the paint has been removed (bare metal), and the surrounding, intact painted area. The two outer legs are firm, while the legs in the centre are connected to an indicator and scale. The instrument, which can be used for this, is called a dial gauge. This method is often used in shop priming plants for DFT measurements on flat steel or glass plates.
The paint inspection gauge, P.I.G..
Another method for determining the film-thickness is with a paint inspection instrument, also known as the P.I.G., which is an abbreviation for Paint Inspection Gauge. This instrument is equipped for a number of different functions, including three knives, which can be used to bevel the paint film. By this method and with help of a scale in a little microscope at the end of the apparatus, the film thickness of one or several coats of paint can easily be determined.
8.5.1.3. Calculation of Dry-film Thickness (dft)
It is important not to exceed the minimum and maximum DFT given by the paint specification. Also, the manufacturer’s recommendations should always be followed.
The specification of the average thickness should be sufficiently high to obtain a minimum target DFT. This means that a minimum of 80 % of all thickness measurements ought to be > the specified mean DFT. Of the remaining 20 % of the measurements, none should be below 80 % of the specified DFT (this is called the 80/20 practise). Example: To obtain a minimum DFT of 200 µm the average or nominal DFT should be specified to be at about 250 µm.
DFT can be estimated from the WFT. A volume per cent solid is found in the technical data sheet.
DFT = WFT x (Volume % solids )
100
See also section 16 “Calculations” regarding coating related calculations.
8.5.2. Curing / drying.
An inspector might be required to determine whether or not a coating is sufficiently cured. Often the inspector will be relying on the data sheets and the daily logs concerning the coating application.
Pencil hardness test can be used for the control of curing. A butt-ended pencil is held at an angle of 45o to the substrate and is pushed into the coating. Start with the softest pencil and then increase the hardness until you find one that scratches the paint film.
Another test can be the use of special hardness testers that are held to the surface and a spring-loaded plunger is driven into the coating.
Sometimes for two pack paints such as epoxy and polyurethane coatings a solvent resistivity test is used. There are different methods. One method is to saturate a cloth/rag with solvent. The cloth/rag is wiped across the surface of the coating for a specified number if times. After completion of the rubbing the cloth or rag is inspected visually. Very little, if anything at all, shall be visible if the paint is cured.
Curing of zinc ethylsilicate paints.
The test method for testing the curing of zinc ethylsilicate paints is standardised as ASTM D 4752 - the MEK test. The surface is cleaned with freshwater prior to the test to remove all loose zinc dust. A white cloth/rag is saturated with the solvent Methyl-Ethyl-Kethone (MEK) and rubbed over a selected area of at least 50 mm. Use the thumb and perform up to 50 double rubs across the zinc ethylsilicate. If the solvent has very little or no effect on the surface, then the coating will be rated as 4 or 5 according to the standard. At this level one assumes the coating to be cured.