Many of the specifications and data sheet we use today are converted from the United States who use imperial units like mils/thous (1/1000 of an inch = 25.4 microns).

When we measure profile and the specification has been converted from an Imperial standard for the profile reading (for example 2 ½ mils), to the equivalent metric standard (ie 63 microns) it’s a little bit like comparing analogue time with digital time. One day we say “I’ll see you at half past 2” and in reality if the meeting started at 2:26 or 2:34 no-one would really mind. But if it’s digital you say it’s starting at 2:30 and 2:29 is early and 2:31 is late.

Applying that to profile, one of the most common methods for measuring surface profile, is Testex replica tape. If the specification calls for 3 mils and the actual reading is 2¾ mils, it is only 1 increment away on the gauge face and the inspector may say “it’s close enough I’ll pass it.”

If you have the same reading in microns, and the specification says it must be 75 microns (3 mils is about 75 microns), if a reading of only 69 microns is achieved (the equivalent to 2 ¾ mils), well that reading hasn’t even got a 7 on the front and the inspector may be likely to say “no, it’s a fail and it needs to be re-blasted to get a higher profile.”

These challenges that confront the industry are merely pointed out as areas that leave clients, contractors and inspectors in a position where they feel they need to make their own interpretation. They are making on the spot decisions that may appear right at the time with all good intent, but have the hidden effect of costing someone, often the contractor, a lot of money, unnecessarily.

I would like to refer to instances five instances of research in this field. Although the research projects referenced were not conducted specifically for this exact issue, they contain results that are of interest and relevance around the question of whether higher surface profile increases coating adhesion.

1. In 2006 CTI Consultants undertook some testing around blasting with different grades of garnet. Four different grades were used. Some of the results of the research are shown below and there is no clear indication of any difference in coating adhesion across three different types of coatings, whether they are applied on a 30 micron profile through to a 60 micron profile.
2. In 1983 Corrosion Control Consultants and Labs working with the Michigan Department of Transport concluded there have been more coating failures due to excessive profile than low profile. Bearing in mind, that is in contrast to no profile. It assumed the steel had been blasted to create some profile but the profile was low.
3. In 2005 authors Roper, Werner and Brandon investigated the effect of the peak and valley count in the profile, commonly called the Peak Count, which is simply a measurement of the number of peaks and valleys in the profile. The results of their investigation is very interesting. There are two outcomes from their research:
   1. One is that across different profile heights there was no effect on adhesion.
   2. Secondly, the peak count, that is the number of peaks and valleys in the profile does have an effect on adhesion. The interesting thing about that is it is generally assumed that to increase your peak count you actually have a smaller profile because you’re using a smaller abrasive.
   3. In addition, they say a consistent profile height will provide better coating performance.
   4. Higher peak count generally means low surface profile reading.
4. In 2011 Extrin Consultants were looking at the adhesion of coatings over surfaces blasted with six different abrasives. It is quite an extensive paper with a large database of results. In those results there is no correlation of a greater coating adhesion being achieved on a surface with greater surface profile.
5. In 2008 Darren Ward from International Paints in the UK was investigating profile shape, that is the profile produced by angular grits versus rounded steel shots and concludes that there is no significant differences in performance when comparing different grades of shot or grit.

To try and get an answer to the question whether profile height affects adhesion we decided to use a single abrasive type to a universally accepted cleanliness of blast. The same coating with three different profile heights and three different coating thicknesses, all with a solvent less epoxy coating. We used Testex tape to measure the surface profile. The results shown in the table with surface profiles of 25 microns, 75 microns and 125 microns surface profiles across and utilizing coating thicknesses of Dry Film Thickness of 250 microns, 500 microns and 1,000 microns.

It is very interesting to note that a 1,000 micron coating on a 25 micron surface profile had very similar adhesion results to a 250 micron coating on a 125 micron surface profile. This leads us to conclude that there is no noticeable correlation and verifies that there is no statistical relationship, between profile height and coating adhesion.

The results also showed the limitations of utilizing Testex tape over a wide range of profile heights requiring the use of different Testex tape ranges and the variation between the different grades of Testex tape.

To calibrate the gauge for a 250 micron coating, 300 micron shims were used to do a rough surface calibration to try and get an accurate coating thickness reading which was the full thickness of the coating above the tops of the peaks of the surface profile.

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