• Overcoating of shopprimers?

Overcoating of shopprimers?

Not seldomly, newbuild steel structures are 'pre-blasted' (manually or automatically) and protected with a (cheap and/or water-based) shop primer.

Is it wise to overcoat this shop primer with a more performant Epoxy- and/or Polyurethane coating system?

Some tips and ideas perhaps...

When a waterbased shop primer needs to be overcoated with a more performant Epoxy/Polyurethane coating system, we recommend taking the following precautions:

1) Will the shop primer be removed or overcoated later on?

The first question you need to ask is whether or not it is deemed acceptable or desired that the shop primer will remain present and be overcoated later, or if it will be removed before the application of the final corrosion protection?  This is very important in terms of compatibility during the selection of the type of shop primer to use.  See also below.

2) How long is the shop primer required to provide for (temporary) corrosion protection, and in what type of environment?

Certain types of shop primers can provide for 6-9 months of temporary corrosion protection in a not-too-corrosive environment.  However, often this period will be shorter.  The type of resin, the pigmentation as well as the thickness of the shop primer are decisive factors in this.

3) Compatibility

If the shop primer will be overcoated with a more performant definitive coating system, then it is advised to verify if the coating system to be applied later (often solvent based) is 'compatible' with the shop primer (to be) used!

Ideally, both shop primer and the rest of the coating system are sourced from the same coating manufacturer/supplier.  This way, your manufacturer/supplier can ensure that your coating system is fully compatible.  Oftentimes however there are unknowns or factors beyond your control, especially when it comes to shop primer...

When a coating system from manufacturer X needs to be applied to a shop primer of manufacturer Y, it would be cautious to contact both manufacturers and ask them for their (preferably written) confirmation of mutual compatibility (specifying all products, thicknesses, total coating-system, surface-preparation, etc...)

When such information or confirmation is missing, then Table B.1 from standard ISO 12944-5 can be a good 'guideline' (see attached illustration).

Do take into account that this concerns a 'general/generic' approach and can differ in certain specific cases.  If you still wish or need to apply a combination against this guideline, then we typically recommend performing a so-called 'compatibility-test'.

ASTM D5064 can be instrumental in doing so.  This standard foresees a 'trial-application' (in case: solvent-borne coating-system applied onto waterbased shop primer), on which, after sufficient curing (typically > 7 days) a verification is done of the adhesion of the coating (see also ASTM D3359) and the test-patch is examined for other coating-defects such as but not limited to wrinkling, blistering, checking, mud cracking, checking, cracking, peeling, lifting, disbonding, ...

Such verification could easily take a couple of weeks to complete, however, if the location, size, or use of the structure to be coated is anywhere important or critical, then it's always "better to be safe than sorry", right?

4) Surface Preparation

And when the shop primer is properly selected and found to be compatible with the full coating system, it will quite likely also be a good idea to ensure that the surface preparation is what it should be (for the complete coating system)!

Materials to be shop primer are often 'pre-blasted' in an automated blasting cabin using 'shot'-abrasive (round).  This typically yields a blast profile suitable for the proper adhesion of the shop primer, however depending on the nature and thickness of the full coating system, may not be suitable anymore.  This may cause the full coating system to still detach from the surface, where initially the shop primer was showing proper adhesion.  Also, note that not just the 'type of abrasive' (shot vs angular) is important, but also the anchor-profile as well. If the shop primed structure will be exposed to an outdoor environment for a certain period, then the anchor profile should not be too high (± 20 - 40 µm).  However, this may be insufficient for the (thicker) final coating system that will be applied later on...

See also:

  • ISO 12944-5, Tabel B.1
  • SSPC-PS Guide 22.00 Guide for Selecting One-Coat Preconstruction or Prefabrication Painting Systems (www.sspc.org)

In conclusion:

When no information is known about the surface-preparation quality (cleanliness, roughness, ...), the shop primer can possibly be removed locally to still make an accurate assessment of this important foundation of any coating system.

All of this will likely cost you a couple of hours of work, investigation, and testing, ... to obtain optimal confirmation.  However, this small investment can easily save you several days, weeks, ... or more if a coating problem arises at the building site, or worse after the structure has been taken into service...

We are fully equipped and experienced to assist you with such analysis should you need any assistance!

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