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March 10 - March 14, 2014

What do you do when the surface profile is higher than the dry film thickness of the specified primer?

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Selected Answers

From Christian Favennec of DCNS on April 22, 2014:
The nominal surface profile to achieve before application is defined in the technical data sheet of the paint.Generally, it's defined as medium grit according to ISO 8503-1 (Ry 50 to 84 µm ; 2.0 to 3.4 mils). But it can be lower (fine, Ry 23 to 49 µm) or higher (coarse, 85 to 129 µm), depending on the primer to be applied.The dry film thickness specified corresponds to the film thickness over the profile peaks. ISO 19840 defines two method for measurement of dry film thickness on rough surfaces: either by calibration on a smooth surface and taking a correction value on the measurement (25 µm for medium profile, 40 µm for coarse profile. The other method consists of the direct calibration of the DFT gauge on the rough surface. If the roughness achieved is in accordance with the recommendation of the technical data sheet, it means that the paint is formulated to be applied on this profile. If the profile is over the recommendation, the TDS also gives  information on the range of DFT acceptable for the paint, and it's possible to check, with confirmation from the supplier, if the "extra-thickness" is acceptable for the paint. This extra thickness can be a problem with some paint, such as ethyl silicate zinc, which can suffer cracking; or low-solids paint, which can flow and not fully cover  the peaks. If the profile is acceptable, then the DFT can be checked according to ISO 19840. It will result in extra paint consumption. If the profile is not acceptable, either the surface must be re-blasted to lower the profile, or the  primer has to be changed.

From juan hurtado of International Paint on March 20, 2014:
Of all this jibber-jabber I just read on what steps to take or whom to blame, I never did read anybody asking if the specs defined using coarse or extra coarse profile tape. It really chaps my behind reading the first question asked. I have been in this same situation with other inspectors making a big fuss over the same matter, when  the inspector was mistakenly using the incorrect size tape, coarse instead of extra coarse.

 If the steel does have a deeper profile than the depth specified, the contractor is going have to eat the costs of the extra coating applied to cover the peaks, a simple fix. It seems the person who posted the question has little experience and is going by the book, which is a guideline we can follow. I'm not saying to cheat; that's not what I am suggesting. One of the other comments was to reblast with a smaller grit profile to maybe achieve a profile that would meet the spec. But this would require more man hours and material. While it might solve the problem,  at what cost?

From Aldrin Cordovez of Kuwait Oil Company on March 18, 2014:
I consider this scenario from a coating inspector’s perspective and the steel is new, not previously coated. Once a coating inspector found that surface profile is higher than the specified thickness of the primer, he should first check thoroughly the technical details of the Product Data Sheets (PDS)  to see if some guidelines are available in this regard. The next step would be to consult the coating manufacturer since some blast primers are actually designed to cover excess peaks of the blasted surface, such as thick coating systems like glass flake and fiberglass lining systems for tank internals. Another possibility is to change the coating system using high build epoxy, if the situation permits. The last thing the coating manufacturer could advise is to re-blast the entire surface using the most suitable type of abrasive, considering the hardness, particle shape/size, angle of nozzle, and blast pressure to ensure that this blast process is meeting the specified profile. A test re-blasting should be carried out to determine its effectiveness.

From William Slama of International Paint/Ceilcote Products on March 13, 2014:
I don’t see that as a problem as long as the primer is “able” to cure or dry properly at greater thickness. That is probably not a problem for 100% solids system and may be OK for high solids systems. In that regard, the coating manufacturer should be consulted. However, the main thing to remember is that the correct primer thickness measurement, wet or dry, is the thickness above the top of the profile. That is why any dry film thickness measurements should be done with the instrument calibrated on the profiled surface. For low solids primers over high profiles, it is sometimes possible that the dry primer does not maintain even enough volume to fill the profile. In those cases the “wetting” ability of the primer is required just to retain thin film over the peaks. When clear primers are used, this sometimes will result in visible staining (rust) if there is high humidity and the primed surface is not top-coated soon enough. It would then have to be re-blasted and re-primed. An important factor that this identifies is that the profile must be considered when calculating the expected primer coverage (sq. ft. per gallon). Let’s consider a 4-mil profile and 100% solids primer specified at 2-4 mils thickness. Neglecting shrinkage, the primer must cover the profile by 2 mils, but it needs to “fill” the profile. The simplest approximation for the added primer requirement is that the volume below the tips of the profile is about ½ of the profile depth. So if an average of 3 mils is specified, one has to add 2 more mils of primer to fill the 4 mil profile. So for 3 mils of 100% solids primer over the 4 mil blast profile, the coverage will be 1604 mil*sq. ft./gallon divided by (3+2) mils = 321 sq. ft. per gallon. Note that the actual volumetric shrinkage of the primer must be factored in also, as well as expected waste.

From Bryan Zofkie of Aerco Sandblasting Co. on March 12, 2014:
Sometimes, this issue can be a little more complex than simply choosing the correct media. There are a lot of older and aging steel structures in the field these days, so there can be a little complexity added because of this. As Mr. Brunner said, choosing the correct abrasive from the start in order to provide the correct surface profile is paramount, and on new steel there should never be a question of what profile you are going to get. On older structures there are many variables that come into play, such as the existing profile. It is possible that the structure has been abrasive-blasted at some point prior and has a larger existing profile because of that. Another variable could be an existing coating that needs to be removed. Some coatings, especially those of greater thickness, can be efficiently and economically removed only with larger blast media. This, in turn, could leave you with a larger profile than desired for the new coating. As mentioned above, this would be the time to consult the coating manufacturer. More times than not a simple resolution can be achieved.

Contact the paint representative and seek direction on remedial actions. This is important to ensure that quality work will prevail.

From Steve Brunner of WPC Technologies on March 11, 2014:
There are two issues here: the first is that project management used the wrong abrasive or abrasive set-up. This is the easy fix as a pre-job conference will correct future problems. The second and immediate problem is the current too high profile. As Cristiano Godoy mentioned, contacting the manufacturer for a letter of accepting out of spec profile is one way. Most manufacturers will agree. Another  fix is to reblast the surface. Here, one should have the nozzle at a 45° angle then inspect the profile. Profile may have bent peaks. Often after the 45° blast, a "normal blast technique" is performed with a proper grit abrasive. While this fix is a little labor-intensive, it will get you back on track. REMEMBER: plan ahead so you can avoid problems.

From Cristiano Godoy of MB Environmental on March 11, 2014:
First of all, I would contact the paint manufacturer. For example, if after abrasive blasting, my profile on the steel is 5 or 6 mils, and I need to apply a  zinc coating at 3-4 mils, according to the manufacturer's product data sheet,  I would call the manufacturer, asking for a letter allowing the use of his product in this situation.

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Tagged categories: Dry Film Thickness (DFT); Primers; Specifiers; Surface profile

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