A coating can only be expected to provide the projected protection if the dry film thickness (DFT) meets the specified/recommended film thickness.

A. Wet film thickness (WFT)

By controlling the wet film thickness during application, we will also control the dry film thickness, provided no unauthorised thinner has been added to the coating. When we know the %VS, we can easily calculate the resulting DFT from an applied WFT. Refer to ISO 2808 Method 1A and 1B for how the WFT shall be measured.

Comb Gauge ISO 2808 Method 1A

Comb gauge is a plate with a lot of teeth on its edge(s). Those teeth have various height indicating WFT. The comb gauge can be made from different materials, such as stainless steel or solvent resistant plastic (disposable type). Comb gauge is the most popular tool onsite.

Procedure

1. Select appropriate side of gauge
2. Insert gauge vertically into wet film just after movement of nozzle
3. Observe two neighbouring teeth, one with paint and the other without
4. The actual WFT is in between those two teeth
5. Practically the highest tooth with paint is identified as the WFT

 

Wheel Gauge ISO 2808 Method 1B

• Wheel gauge consist of a wheel with two ground rims in one axes, and a third middle eccentric rim with a slightly smaller diameter. Scale on one of the ground rims can indicate result of WFT.

 

Procedure

• Insert gauge vertically into wet film just after movement of nozzle
• Roll the wheel in the wet film until a break in the picked-up paint shows on edge of the eccentric rim
• Record the figure on ground rim where break is located

 

For all WFT readings

• Measurement must be taken immediately after all passes are finished
• Accuracy is affected by fast evaporation solvent containing paint, such as zinc rich paint
• In multi-layer physically drying system, the methods can work well for first coat but not subsequent coats
• Clean the gauge with solvent every time after use
• The gauge is applicable only on flat substrates

 

B. Dry film Thickness (DFT)

Methods to measure DFT can be categorized in two groups: non-destructive and destructive. There are also different principles and instruments used for various substrate and coatings.

• Non-destructive:
  • Magnetic Pull-off (Ferrous substrates)
  • Magnetic induction (Ferrous substrates
  • Eddy current (Non-ferrous substrates)
  • Ultra sonic (Non-metallic substrates)
• Destructive:
  • I.G. (Paint Inspector Gauge) (Any substrate)

 

Non-destructive: Ferrous

Magnetic Pull-off Gauge ISO 2808 Method 7A

• Mechanical without battery
• Simple to operate
• Not very accurate

 

Magnetic Induction gauge ISO 2808 Method 7C

• Low-frequency electromagnetic field
• Various verification methods for accuracy
• Some with internal memory and printing function 

Using a magnetic induction DFT gauge

• Used for non-magnetic coatings on ferrous substrates
• Verify gauge according to manufacture recommendation or referred standards
• Check with your calibration foil frequently
• Plastic material foils wear easily, replace frequently
• Newer models have internal memory, statistical functions and Bluetooth transfer facility of data to computer and smart-phone

 

Factors influencing accuracy

• Standards to be referred
• Thickness & composition of substrate
• Structure of testing area (surface profile)
• Edge effects
• Vibration
• Calibration
• Verification method
• Frequency and extent of readings taken
• Acceptance/rejection criteria

 

Checking DFT

 

Non-destructive: Non-Ferrous metals

Eddy current gauge ISO 2808 Method 7D

• High-frequency electromagnetic field
• Commonly used on non-ferrous substrates
• For aluminium, copper, stainless steel, etc.

 

This gauge can be used for non-conductive coatings on electrically conductive substrates. Method to use gauge is very similar as magnetic induction gauge

 

Non-destructive: Non-metallic

• Paint is also applied on non-metallic substrate, such as concrete, plastic, wood, etc.
• Purpose to apply paint on such materials are:
  • Protection
  • Decoration
  • Signal
  • Fire fighting
  • Prevent bacteria
• These film thickness gauges are based on ultra-sonic principle

 

Destructive DFT Measurement – P.I.G.

• Paint inspection gauge (P.I.G.)
• It is a destructive method that can measure dry film thickness for each individual coat in a paint system
• Repair must be carried out on the spots where measurement has taken place
• I.G. is not used in daily inspections
• The use of P.I.G. is described in ISO 2808 Method 5B

 

Apparatus

• Marker pen: colour contrast with topcoat
• Magnifier: 50 x
• Blade: cut film to form a surface for observation

 

How to Use P.I.G.

1. Take the coated product
2. Using the marker pen, make a line across the coating
3. Using the PIG, make a cut at right angles to the marker line, all the way down to the substrate
4. Using the microscope, measure the number of divisions across a coating layer, using the sharp cut
5. Using the Scale factor (number of microns or mils per division), calculate the coating thickness of each layer in the coating system

 

Remember that the Scale factor is depending on the angle of the V-cut made by the blade:

 

Blade No.	Angle	Range (um)	Factor (um)
No. 1	45°	20 - 2000	20
No. 4	26.6 °	10 – 1000	10
No. 6	5.7 °	2 – 200	2

 

Common Standards for DFT Measurement

Various standards are used within the industry:

• ISO 2808 Paints and varnishes — Determination of film thickness
• ISO 19840 Paints and varnishes - Corrosion protection of steel structures by protective paint systems - Measurement of, and acceptance criteria for the thickness of dry films on rough surfaces
• SSPC PA2 Measurement of Dry Coating Thickness with Magnetic Gages

 

Inspector can refer to above standards for:

• Verification of gauge
  • Has it been set to zero on smooth steel or on steel with a blast profile? Should you use a corrective value?
• Sampling plan
  • How many DFT measure should be taken in a given area?
• Measurement
• Acceptance / reject criteria
  • 80/20 rule
• Test report

The adhesion of a paint film to its substrate is probably the most important factor for a successful result.

Two types of adhesion testing are being used in field:

• Cutting by sharp blade (Cross-cut, X-cut)
• Pull off

These are described in the following standards:

• ISO 2409
• ISO 4624
• ISO 16276

Cross Cutting test

Principle

Six parallel cuts are made in the paint film using a sharp blade and another six cuts are made at 90^o angle to the first six. Adhesive tape is applied over the intersection of the cuts and pulled off. Evaluate how much of the paint was removed by the tape.

Two types blade can be used:

• Single blade
• Multi-blades

ISO 2409

• This method is appropriate to DFT up to 250 µm
• Six cuts are specified in ISO 2409
• Space between each cut is based on DFT of coating system:

Procedure

1. Cut the coating down to substrate with appropriate spacing guide
2. Repeat this operation, making further parallel cuts of equal number, crossing the original cuts at 90°angle
3. Brush the panel lightly with the soft brush
4. Rub the tape firmly with a fingertip or fingernail
5. Remove the tape by angle 60°
6. Evaluate the results with illustration in standard

X Cutting test

Only a single blade is used for this test

Procedure

1. Make two cuts, resembling an “X”, 40 mm long
2. They should cross each other near their middle, at an angle of 30 – 45°
3. Apply adhesive tape at intersection
4. Pull off rapidly (not jerked) at 180^o angle
5. Inspect for removal of coating, rate in accordance with scale in standard

Assessment – ISO 16276

Level 0 - No peeling or removal of coating

Level 1 - Trace peeling or removal along cuts or at their intersection

Level 2 - Jagged removal along cuts, extending up to 1,5 mm out on either side

Level 3 - Jagged removal along most of the length of the cuts, extending up to 3,0 mm out on either side

Level 4 - Removal from most of the area of the X-cut under the tape

Level 5 - Removal of coating beyond the area of the X-cut

Pull-off Testing - Apparatus

Principle

Small metal “dollies” are glued to the paint film and pulled off with a special instrument. The pull off force is measured and is a measure of the paint’s adhesion.

Equipment

• Pull off tester:
  • Hydraulic
  • Pneumatic
  • Manual
• Dolly
• Adhesive
• Blade

Procedure

1. Check DFT and climatic condition, record
2. Clean the surface of testing area and dolly with sand paper
3. Apply glue onto dolly and attach it firmly to test area
4. Let glue cure / dry
5. Cut surrounding film of dolly down to substrate
6. Pull off dolly with appropriate tester
7. Record fracture strength
8. Analyse failure type, location and area percentage
9. Make report

Analysing testing result

Study the two surfaces where the adhesion break happened and identify what the measured pull-off force is related to:

• Adhesion between paint and substrate
• Adhesion between different coats within the paint system
• Cohesion within a coat of paint
• Adhesion between the dolly and the paint (glue strength)
• A combination of some or all of the above possibilities

The break will usually be a combination of different breaks. All involved parties should agree on acceptance criteria corresponding to different failure types. It is important to identify recurrences of different failures in a paint system.

What should be reported?

• Breaking strength
• Nature of fracture
  • Adhesive failure
  • Cohesive failure
  • Glue failure
• Location of failure
• Percentage of each failure
• Inspection date
• Inspector name
• Standard to be referred
• All details of substrate
• All details of surface preparation
• Description of sampling
• Acceptance criteria
• Ambient condition
• All details of system to be tested
• All details of apparatus to be used

The type of break (and their percentages) should be reported as described in ISO 4624.

Holiday detection

Two methods to detect holidays in film by different standards:

• Low voltage (wet sponge) detector
• High voltage (spark) testing

Low voltage (wet sponge) testing

• Refer to ISO 29601 or NACE SP 0188 or ASTM D 5162 Method A
• It is used to locate discontinuities in a nonconductive coating applied to a conductive substrate
• Appropriate to dry film thickness up to 500 µm
• Using a wetting agent can increase sensitivities of test
• However, if a wetting agent is used, it must be completely removed by rinsing the holiday area prior to repair
• It is a non-destructive testing

Procedure

1. Check DFT to select proper voltage if necessary
2. Connect clamp to bare steel
3. Wet sponge with conductible water
4. Move sponge onto test areas with speed 0,3 m/s by double passes
5. Once gauge beeps, turn corner of sponge to find exact location of holidays
6. Record

Factors influencing measurements

• Topcoat must be non-conductible
• Tested coating(s) must be fully dried
• Surface should be free from contamination
• Total film thickness should less than 500 µm
• The substrate must be conductible
• Sponge shall be wetted “sufficiently to barely avoid dripping”

High voltage (spark) testing

• Refer to ISO 29601 or NACE SP 0188 or ASTM D 5162 Method B
• Appropriate to dry film thickness above than 500 µm
• This method can detect:
  • Holiday
  • Low film thickness
  • Discontinuous in film

Procedure

1. Check DFT to select proper voltage
2. Connect clamp to bare steel
3. Move electrode onto test areas with speed 0,3 m/s by single pass
4. Once gauge beeping and see spark, remove electrode from testing surface and mark for repairing

Factors influencing measurements

• Topcoat must be non-conductible
• The coating(s) must be fully dried. Otherwise, spark may ignite solvent (vapor)
• Surface should be free from contamination
• Generally, this method is aiming at thicknesses exceeding 500 µm
• The substrate must be conductible

Determine curing of Ethyl Silicate Zinc-Rich Primers

• Inorganic Zinc ethyl silicate (IOZ) is a moisture curing paint
• If over coated before fully cured, the IOZ will lack moisture and will never cure
• It is important to identify curing degree prior to over-coating
• MEK rubbing according to ASTM D 4752 is a common test method in the industry

ASTM D 4752:  Standard Test Method for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub

Procedure

1. Clean the testing area with clean cloth to remove loose dust
2. Fold a clean white rag to double size
3. Saturate rag with MEK
4. Double rub the test area for 50 times or see substrate exposed
5. Find an adjacent area to do same procedure but with dry rag
6. Compare two rub areas and grade

Classification of result

5 - No effect on surface; no zinc on cloth after 50 double rubs

4 - Burnished appearance in rubbed area; slight amount of zinc on cloth after 50 double rubs

3 - Some marring and apparent depression of the film after 50 double rubs

2 - Heavy marring; obvious depression in the film after 50 double rubs

1 - Heavy depression in the film but no actual penetration to the substrate after 50 double rubs

0 - Penetration to the substrate in 50 double rubs or less

Identify hardness of paints

• Pencil hardness testing is a simple field testing method according to ASTM D 3363
• Other, more accurate laboratory test methods are available

Apparatus

• Pencils: 6B-6H
• Mechanical Lead Holder

Procedure

1. Prepare the lead in the pencil as described in the standard
2. Fix the hardest pencil (6H) to Mechanical Lead Holder
3. Push pencil away from operator on test surface
4. If the tested paint shows cut or scratch or crumble of lead, use the next hardest pencil
5. Continue until a pencil do not cut the film, record hardness of the pencil as film hardness

Report

• Gouge Hardness: The hardest pencil that will leave the film uncut for a stroke length of at least 3 mm (1⁄8 in.)
• Scratch Hardness: The hardest pencil that will not rupture or scratch the film

Solvent test

• Coatings will have different solvent resistance depending to their drying/curing process
• We can determine drying/curing process of an already dried/cured coating by using the solvent test
• Based on the test result, a compatible repair system can be specified

Procedure

• Place a piece of cotton or textile on the test surface and saturate this with strong solvent
• Cover the test area with a lid to reduce evaporation
• Wait for reaction about 20 minutes
• Observe the surface condition:
  • Dissolves – physically drying paint
  • Wrinkles – oxidatively curing paint
  • No reaction – chemically curing paint

ISO 4628 Evaluation of degradation of coatings

Facilitates identification of different types of defects in terms of:

• Intensity
• Quantity
• Size

The standard ISO 4628 consists of nine parts:

• Part 1: General introduction and designation system
• Part 2: Assessment of degree of blistering
• Part 3: Assessment of degree of rusting
• Part 4: Assessment of degree of cracking
• Part 5: Assessment of degree of flaking
• Part 6: Assessment of degree of chalking by tape method
• Part 7: Assessment of degree of chalking by velvet method
• Part 8: Assessment of degree of delamination and corrosion around a scribe
• Part 10: Assessment of degree of filiform corrosion

Intensity

Used for uniform deterioration (e.g. chalking)

• Rating scheme for designation the intensity of deterioration consisting of a uniform change in the visual appearance of the paint coating

Quantity

Used for scattered defects (e.g. blisters, cracking, rust-penetration)

• Rating scheme for designating the quantity of defects consisting of discontinuities or other local imperfections of the paint coating

Size

Rating scheme for designating the size (order of magnitude) of defects

Report

• All details necessary to identify the coating examined
• A reference to the relevant part of ISO 4628
• The type of surface examined, its size and, if appropriate, its location
• The result of the assessment
• An indication of the illumination under which the assessment was carried out
• Any unusual features (anomalies)