Surface Preparation of Metals – Profile - Tips and Tricks:

NOTE: The following comments are the opinion of the author based on experience using the following equipment in both a laboratory and field environments. Any comments are welcome and can be forwarded to me at tswan@cci-inspection.com

Surface preparation can be broken down into two main categories.

• Surface Profile
• Surface Cleanliness

Surface profile is the determination of the roughness of the surface and for painting purposes involves depth of the profile and angularity of the profile.

Surface Cleanliness involves determining how much of the original mill scale, rust and paint have been removed from the surface as well as how much invisible surface contamination is present usually in the form of salts. More on Surface cleanliness can be found in the Surface Preparation – Cleanliness - Tips and Tricks on the menu.

Surface preparation is probably the main cause of most paint failures because more than any other factor, it affects how well the paint sticks to the surface being painted.

I will attempt to explain the importance of surface preparation by comparing it to climbing a mountain.

There are two factors that will determine if you reach the top of the mountain. How smooth is the surface (the substrate) and what type of equipment are you using (the paint)

1. You are climbing the mountain and the surface is covered with ice. Since the surface is smooth, you need equipment like a pickax that will hold onto the ice to be able to climb.
2. You are climbing the mountain and the surface is smooth with some cracks and crevices. Since the surface is smooth, you need equipment that will get into the cracks to be able to climb.
3. You are climbing the mountain and the surface is rough with many hand holds. A relatively skilled mountain climber can get up the mountain.
4. You are climbing the mountain and the surface is very rough with many rocks and boulders to climb on. Anyone with a minimal amount of effort can climb the mountain.
5. You are climbing the above mountain (4.) but you find there are many deep crevices (Corrosion and pitting). Without the proper equipment, the climb has become dangerous.
6. You are climbing the mountain in 4. again but instead of crevices there is a lot of loose rocks and dirt (rust, mill scale and loose plants). If you aren’t careful, you will start a landslide.

Hopefully, the above example explains while profile is important. The main purpose of the above example is to point out that the rougher the surface profile, (not counting crevices and loose rocks) the easier the climb. Smooth surfaces can be painted, but make sure the paint is made for that type of surface. Most surfaces can be coated successfully; however, in general, the rougher the profile, the better the paint job.

Most paint forms a mechanical bond with the steel and generally surfaces that have roughness will supply the best mechanical bonds. Also, when you put a profile on the surface, you increase the surface area, so the paint has more surface to adhere to. Different paints are made for different texture surfaces from smooth to rough.

Also, remember, paint will bond to the surface being coated and if the surface is loose (rust, mill scale or old paint), when the surface breaks off, so will the paint. Some paints are formulated to coat over these surfaces with minimal surface preparation, but they should be used with caution and understanding

On Metal, is there a difference in profile due to sand, shot or grit?

The above drawings are rough approximations of the type of profile you might get from Sand, Shot or Grit. The profile can vary due to many different factors, however, generally sand has a finer profile than grit and shot gives a rounded, “pinged” type of profile. The above drawings all have about the same profile depth, but have an entirely different appearance.

While it is rarely measured for industrial coating applications, the peak density can be an important factor in determining the bond of the coating to the substrate. In general, sand will have the “highest” peak density and shot the “lowest”. When a specification calls for an “angular” surface profile, this is generally best done with Grit.

What is Surface Profile Depth.?

An “exact” profile depth can not be determined over the surface of the substrate because the depth of the Peeks and Valleys varies greatly. Surface profile attempts to find the AVERAGE depth for the peaks and valleys over a given area.

What is the best way to find the Profile Depth?

There are currently four accepted ways to find surface profile and each one has advantages and disadvantages. The First Three methods are detailed in ASTM D4417, “Standard Test Method for Field Measurement of Surface Profile of Blast Cleaned Steel”

Surface Profile Visual Comparator (D4417 Method A)

1. Surface Profile Gage – Profilometer (D4417 Method B)
2. Press-o-film Testex® Tape (D4417 Method C)
3. Surface Roughness Tester

Surface Profile Visual Comparator: - Method A

There are several different visual comparators. Elcometer sells: The Elcometer 125 Surface Comparator, The Elcometer 129 Rubert & Rugotest Comparator and the Elcometer 127 Keane-Tator Surface Comparator.

These comparators are cast in metal to approximate surface profiles up to 4 mils. The comparators are held up to the blast profile, and usually viewed with a lighted magnifier (5X or 10X) and the texture and roughness of the comparator is compared to the blasted steel. Because the profile changes based on the media used, comparators are sold as Sand, Shot, or Grit. Comparators, when used by an experienced inspector should give an accuracy to approximately ±0.5 mils within the range of the comparator. Since not all grit gives the same profile, this is probably the most problematic comparator to use. Until the operator is experienced, it is usually best to “calibrate” their eyes using Testes Tape to confirm readings.

A Surface Profile Gage – Method B

This is a gage with a wide base that sits on the peeks and has a needle that goes into the valleys. Since there is a single point, only one peak to valley reading is made with each reading. To get a good idea of the average surface profile, several measurements must be made and averaged together. The meter must be zeroed to a smooth surface, such as glass. With a dial gage, readings must be recorded as they are made. There is no permanent record.

Elcometer has a new Surface Profile Gauge - the Elcometer 224. This gauge is digital and will record average, min and max readings and download to a computer for a Permanent record. It also has wireless (Bluetooth) built in to the top model.

Press-o-film Testex® Tape – Method C

Testex Tape is probably the most common method used to determine surface profile. The Tape has a compressible foam layer with a 2 mil Mylar covering. A “Burnishing” tool (Most people call this a swizzle stick), is used to rub the foam into the profile. The foam takes on the shape of the profile and it is measured with a spring micrometer. Since the foam is covered with 2 mils of Mylar, this must be subtracted from the reading to get the surface profile.

When using Testex Tape, make sure the area is clean and representative of the area being tested. If the tape is not rubbed with sufficient pressure, the correct reading will not be achieved. As the center of the tape is rubbed, the color changes slightly. The entire surface should look the same.

If the tape is reading the maximum or minimum reading on the Testex tape, the next higher or lower tape should be used. 

The advantages to this method are:

Gives the profile over approximately 3/8 inch area.

1. Gives a permanent record of the test.
2. Easy to do.
3. No objectivity on the part of the operator.

Disadvantage.

1. Can get costly if many measurements are required
2. Improper “Burnishing” of the tape can give low results.
   
3. Since the micrometer reads the thickest area, it will give a number closer to the maximum value rather than the average.

This is a relatively new method and because of its complexities, generally is not used as a field test. The method involves moving a diamond stylus over the surface to be measured. The meter records max and minimum peaks and valleys, number of peaks, peak density on several other variables. The meter can be interfaced to a computer or give a direct printout. These meters start around $1,700 and are generally used in shops that have quality control departments.

Tips and Tricks for using Testex Tape:

· Testex comes in several grades.

The rating of the Testex tape is the maximum rating the tape is guaranteed to read. To find out the maximum Testex tape will read, measure the tape with the micrometer and subtract 2 mils. This is the maximum the tape will read. i.e. if X-course tape measures 8.3 mils prior to use, the tape will read to 6.3 mils.

What if I am on the job and I don’t have a “Burnishing” Tool?

For those of use without a dictionary, Webster’s Dictionary defines burnishing as; “to rub (a material) with a tool for compacting or smoothing”

To acquire the most common burnishing tool, the easiest thing is to break for lunch and go to a restaurant that serves mixed drinks. Request a Swizzle Stick and you have a burnishing tool. It should have a round end and be deburred prior to use. The ASTM method does NOT define what a burnishing tool is. The most common object used is generally the end of a disposable pen.

What is used as a burnishing tool is not as important as to make sure the surface is “burnished” thoroughly. Not rubbing hard enough can lead to erroneous profile readings.

We have a question on a job from several years ago and the tape was included as part of the job record. Will it still read the same?

The answer is; it should. Once the foam is compressed, it should hold the original profile indefinitely.

If the blast profile measures to high or to low. What can we do?

If the profile was measured with a comparator, you could be getting a bad reading due to using the wrong comparator or even if using the correct comparator, the differences between the blast paten on the metal may be to different to get an accurate reading. Use Method B or C to confirm.

If a profile gage was used, make sure it is zeroed properly and if you have a surface with a known profile, test it for accuracy. If it is still off after taking several readings, use Method C to confirm the accuracy of the profile gage. When was the last time you had it calibrated. You might want to send it to Elcometer to have it recertified.

If you used Testex tape, make sure the surface was clean before you use the tape. Dirt on the back of the tape will give erroneous readings. Make sure the tape was rubbed hard enough and with a proper burnishing tool. You thumbnail make not provide sufficient pressure. You can also use Method B to confirm readings however if the tape was used properly, it is the most accurate way to gage profile.

If the blast profile is confirmed and it is still out of spec. Some general trouble shooting ideas may include the following:

A complete discussing of Abrasive Blasting is beyond the scope of this question and a consultant should be contacted if you are having problems.

How does Surface Profile Affect my Paint Usage?

The rule for paint is 1 gallon of paint applied at 1 mil will cover 1,604 sq ft. on a smooth surface.

The effect of blast profile is important in calculating estimates of paint quantities required especially in cases in which the specification requires application of a minimum dry film thickness. For a series of peak to valley heights of an abrasive blast cleaned surface, the greater the peak to valley height, the more paint will be required to fill the profile before a measurable thickness of paint is applied.*

In the above drawings, if Figure #1 represents a 1,604 sq ft area and we needed a 2 mil coating of 100% solids paint, we would need to apply 2 gallons of paint.

It should be fairly easy to determine that Figure #2 has a greater surface area than Figure #1. If Figure #2 is the same area with a Sand or Grit blast profile, You would APPROXIMATE 3 gallons of paint to get 2 mil Dry Film Thickness (DFT). See Rule of Thumb Below.

Rule of Thumb:

A rule of thumb for the determination of the approximate extra paint required to fill a SAND or GRIT blast profile is to multiply the peak to valley height (profile) times 0.5 and add this to the Dry Film Thickness you are trying to achieve..

For example, for a peak to valley height of 10 mils, an additional quantity of paint equal to a full coat at 5 mils dry film thickness will be required.

Because a SHOT Blast Profile is smoother, the amount of paint would be slightly less and you might want to use 0.25 times the blast profile to calculate the additional amount of paint.

*NOTE: See next question for explanation of “before a measurable thickness of paint is applied.”

How Does Profile Effect DFT Measurements?

This is more fully explained in the Dry Film Thickness Tips and Trick, but briefly, When using a DFT Gage the meter must establish a zero point. When you have a surface with peeks and valleys, there is no clear line where the “Zero Point” is. There are areas in the profile that are LESS THAN THE ZERO POINT. The meter will not register any paint as being applied to the surface until it is greater than the zero point.

In the above drawing, if the DFT gage perceives the “Zero Point” to be the dashed line, any paint below the dashed line will not be measured.

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