Coating Integrity Field Repairs That Last

Today, we’ll discuss liquid repairs on FBE (Fusion-Bonded Epoxy). Our guest speaker is Steve Hall, a sales representative at 3M. 

Let’s get right into it. 3M classifies coating repairs into three categories: holiday repairs, small-area repairs, and large-area repairs. Holiday repairs apply to defects that are 4 millimeters or smaller. I’ll explain each category in more detail on the next slide.

A holiday defect is generally not visible to the naked eye; it’s detected with holiday detection equipment. These repairs are typically performed at the coating plant or in the field during electrical continuity testing. A holiday may be caused by a steel defect, a coating defect, contamination, or another issue.

A small-area repair is defined as a defect smaller than 25 square inches. These are usually visible and may result from handling, transportation, or backfilling damage, or from an area an inspector has identified as out of specification and requiring repair.

A large-area defect is bigger than 25 square inches. I’ll explain shortly why small- and large-area repairs are treated differently.

Let’s start with holiday repairs, shown in the upper-left corner, alongside examples of a small-area repair and a large-area repair. I also want to talk about using a template for holiday repairs. In our training, we typically use a template to mask the repair area and guide the abrasion process.

The upper-right image shows an example. We use an 8-mil film on a release liner, placed around the holiday with the defect centered in the middle. I once visited Tenaris in Canada and saw the team using a template. I considered it a best practice because I watched 80 people complete repairs, and every repair looked consistent. You often see poor-quality repairs in the field, so having a template can help improve consistency.

Here are the typical steps for a holiday repair:

1. First, clean the area with solvent if needed. If the holiday has just been identified on a clean pipe at the coating plant, solvent cleaning may not be necessary. But if the pipe is dirty in the field, clean it with solvent. Grinding the defect area is optional. If a steel burr caused the holiday, use a grinder to smooth it out. Be careful not to gouge or otherwise damage the steel.
2. Next, place the template over the cleaned area—or tape off the area if you’re not using a template. At a coating plant, where repairs are often done as free repairs, a template or tape may not be used. In the field, I recommend using some type of masking procedure or template.
3. Then abrade the surface with 80- to 120-grit sandpaper. The goal is to scratch the surface and create a profile, not to gouge deeply into the FBE. You’re aiming to remove about 1 to 2 mils of material. Don’t just lightly touch the surface with the sandpaper; make sure the area is fully abraded.
4. Finally, wipe the abraded surface with a clean, dry cloth. Wipe in one direction so you don’t smear removed material back onto the repair area. Fold the cloth to expose a clean section, wipe again, and repeat a third time. This leaves a clean surface ready for repair.

Next, apply the liquid epoxy coating at a minimum thickness of 25 mils. If the existing coating is 50 or 60 mils thick, apply enough material to match it. Heating is optional. It may be used to speed curing so the repair can be handled sooner. If you heat the pipe before applying the coating, this is typically done with a gas torch. After applying the coating, however, use a commercial heat gun if additional heat is needed.

Remove the template while the coating is still wet, then allow the repair to cure. Cure time depends heavily on temperature. It may take anywhere from about two to 24 hours; four to six hours is common. Once cured, holiday-test the repair and document it.

If the specification allows, a one-part patch-stick product can be used as an alternative for a holiday repair. Follow the product’s application procedure. I prefer two-part systems because they generally provide better adhesion, but some crews use one-part patch sticks.

Small-area repairs
A small-area repair is less than 25 square inches—for example, an area measuring 5 by 5 inches. A template usually isn’t needed, but masking the repair with tape is good practice.

For a field repair, clean the area with a solvent such as acetone. MEK is also suitable, though I typically use acetone because it is less aggressive. Grinding is optional, since small-area repairs don’t usually involve a steel defect.

Mask the repair area as needed, then abrade the surface with 80- to 120-grit sandpaper. You can also use a suitable mechanical tool to roughen the surface. Wipe the abraded area with a clean, dry cloth, as described earlier, then apply the coating and remove the tape.

If needed, use a commercial heat gun to speed curing before the pipe is placed in the ground or covered in the right-of-way. Test the repair before burial, and document the work when required—especially for field repairs.

One note about the order of masking and abrasion: some inspectors want to see abrasion marks outside the masked area. In that case, abrade first, then apply the tape. Other inspectors don’t want abrasion marks beyond the repair boundary. For those repairs, apply the tape first and abrade only inside the masked area.

If you’re using the template shown in the previous example, you’ll abrade inside the template rather than outside it. Inspectors may have different preferences, so follow your company’s procedures and the requirements of the inspectors you work with. My preference is to abrade inside the taped-off area, but some may require the opposite.

Large-area repairs
A large-area repair is anything greater than 25 square inches. For these repairs, we recommend brush blasting rather than sanding. Other companies may use a different threshold for switching to brush blasting; 25 square inches is our typical recommendation.

Start by solvent-cleaning the area. Grinding is optional, but if there’s a steel defect that needs attention, grind it before preparing the surface. Then brush blast the area using steel grit. Clean it with dry, clean air or a lint-free cloth.

Masking is still a good practice, especially for larger repairs. A neat, rectangular repair is generally preferable to an irregular, unevenly shaped one. Apply the coating, and use heat if needed to speed curing. If you’ve used tape, remove it while the coating is still wet. Once the repair has cured, holiday-test the area.

Those are the three repair methods. Before we open it up for questions, I’d like to briefly cover recoating windows.

When repairing a liquid coating, make sure you understand its recoat window. If the coating is still tacky, you can generally recoat over it. For FBE, the surface will need to be abraded before applying another coat. The same applies to liquid coatings once they’re no longer tacky. Follow the appropriate small-area or large-area repair procedure for preparing the surface.

If you’re unsure whether a liquid coating is still tacky, it’s best to abrade the area before recoating.

The 3M products I recommend for the liquid-coating repairs we discussed—including holiday, small-area, and large-area repairs—are 323+ and 328.

Whenever you’re applying a liquid coating, use a repair documentation form and record the work. Document the surface profile, measured with replica tape or a profilometer, as well as the wet- and dry-film thicknesses. You’ll need that information before you can sign off on the repair.

I’m often asked how to tell whether a liquid-coating repair is ready for handling, burial, or final acceptance. The best way to verify that is with a Shore hardness test using a durometer. Many coating products are around 80 hardness, plus or minus three to five points. A reading in the range of 75 to 85 generally indicates that the pipe may be ready to handle.

Also document the holiday-test results and environmental conditions on your quality-control form.

Contractor training is important, especially for crews working in the field. Inspectors may require contractors to show proof that they’ve been trained on the products and application methods they’re using. Training should cover safety and the relevant procedures—whether the crew is performing repairs, roller application, high-cell spray, plural-component application, or other work. Make sure contractors are trained for their assigned tasks and that the training is documented.

QUESTION:
I run into this question when I compare the TDSs: Is the main difference between 328 and 323+ their cured hardness? I understand 323+ is better suited for ARO applications and may offer greater abrasion resistance—is that right?

ANSWER: The 328 is designed for ARO applications and offers greater abrasion resistance. I consider 323+ our standard epoxy, while 328 is our premium epoxy. It can be applied directly to steel and provides better adhesion and flexibility than 323+. It’s also well suited for use as a liquid ARO over FBE.

QUESTION: 
How does the repair perform compared with that factory FBE coding?

ANSWER:  There are several ways to assess the quality of a repair. For example, you could perform an ASTM D4541 pull-off adhesion test. In my view, a good repair should show strong adhesion—potentially withstanding a few thousand psi.

That’s one reason I like the template method and circular repairs: they tend to produce consistently good adhesion. You can also make sample repairs alongside the job and evaluate them by carefully probing the edges with a knife. During a training session in Austin this week, we used that approach to check that the repair edges were well bonded.

When removing the tape or template, it’s important to let the coating flow out at the edges so there’s no sharp, raised edge. That matters especially when the pipe will be buried or used in directional drilling, where rough edges could catch or lift. The repair should blend smoothly into the surrounding coating, with edges that resist being lifted by a knife.

QUESTION:
How does the repair perform compared with the original factory FBE?

ANSWER: Original FBE can easily exceed 3,000 psi in a pull-off adhesion test. We’ve seen FBE bonded to steel reach 10,000 to 12,000 psi. A repair won’t typically achieve that level, but you want it to reach a few thousand psi and demonstrate strong adhesion.

QUESTION:
Can 323+ be used to coat a 30-foot tank, and is it NSF-approved?

ANSWER: Our liquid coatings are no longer NSF-listed, although some other liquid coatings are. In 2018, 3M delisted its liquid coatings. My understanding is that NSF evaluates residuals in parts per million, and the results for two-part liquid epoxies can differ from those for FBE coatings. While our coatings may still fall within the permitted limits, 3M chose to delist them out of an abundance of caution.

I don’t want to overstate the exact reason without confirming the details, but I wish we could still use our liquid coatings in NSF applications.

If you have any more questions, we’ll be sending out a survey to everyone who attended to ensure we’re providing maximum value with these discussions. You can also reach out to us at 248-735-7000 or via email at office@usigroups.com.