Does AWS D1.1:2025 ban welding over primer?

No. The code allows welding over primer provided the coating condition does not adversely affect weld soundness. For prequalified WPS, the primer must be thin and must not impair fusion. For qualified-by-test WPS, the PQR test coupons must replicate the production primer type and film thickness.

How thick can a weld-through primer be under AWS D1.1?

The code does not set a universal mil limit, but the industry-standard threshold for weld-through primer qualification is 2 mils (0.05 mm) dry film thickness. Above that, most EOR specifications and owner standards require removal or a PQR demonstrating soundness at the actual production DFT.

Does zinc-rich primer require a separate PQR?

Not necessarily a separate PQR, but the PQR that supports the WPS must have been welded over the same coating type at the same film thickness. A PQR run on bare steel does not cover production welding over zinc-rich primer.

What should the WPS say about coated base metal?

At minimum: primer type, maximum DFT, and whether the joint zone must be ground to bare metal before welding. If grinding is required, the minimum removal distance from the joint edge must be stated so inspectors can verify it as a hold point.

Weld-through primer on structural steel: WPS documentation requirements

Fab shops welding over zinc-rich or weld-through primer must qualify the coating and document it on the WPS. Here is what AWS D1.1:2025 requires and how audits expose gaps.

Most structural fabrication involves primer-coated steel. The engineer specifies corrosion protection; the coating shop applies primer before the steel arrives at the fab floor; and the welders are now burning through a coating layer on every structural joint. Without proper WPS documentation, this routine process becomes the most common audit deficiency in third-party quality reviews.

AWS D1.1:2025 draws a clear line between welding over an unqualified coating and welding over a tested, documented coating condition. Understanding where that line falls is the job of the CWI or QC manager responsible for WPS preparation.

The prequalified WPS threshold

For prequalified WPS under Clause 5, the code allows base metal surfaces to carry a thin film of mill scale, rust, paint, or primer, provided the condition does not adversely affect weld soundness. "Thin" is the operative word. The code does not fix a universal film-thickness limit in mils, but owner specifications, structural drawings, and project quality plans routinely impose a 2-mil (0.05 mm) dry film thickness (DFT) ceiling for weld-through primer qualification.

Below that ceiling, a prequalified WPS can proceed without a primer-specific PQR — provided the primer type is a certified weld-through formulation (the manufacturer's data sheet must say so) and the WPS acknowledges the coated condition. Above the ceiling, the prequalified route closes and you are in the qualified-by-test world of Clause 6.

How primer condition functions as a WPS essential variable

For qualified-by-test WPS under Clause 6, base metal condition during PQR coupon welding must match production conditions. The logic flows from Table 6.6: essential variables are changes that affect the mechanical or metallurgical soundness of the deposited weld. A change from bare steel to coated steel — or from one primer type to another — changes the fusion environment and can introduce porosity, hydrogen pickup, or zinc embrittlement. These are soundness issues, not cosmetic ones.

The governing principle: if the PQR coupons were welded on bare, cleaned steel, and your production plates carry zinc-rich primer at 3 mils, the PQR does not support that production condition. You either need a new PQR run on coated coupons, or you need a procedural requirement to remove all coating from the joint zone before welding.

Zinc-rich vs. non-zinc-bearing primers

The welding hazard differs by primer chemistry, and how you qualify and document it should reflect that difference.

Zinc-rich primers (organic or inorganic): Zinc volatilizes at welding arc temperatures. The zinc vapor can cause visible surface porosity, subsurface porosity clusters at the root or first pass, and in some high-restraint joints on HSLA steel — zinc-embrittlement cracking along the heat-affected zone. Zinc fume is also a significant health hazard and OSHA-regulated in most jurisdictions. For zinc-rich primer, most owner and general-contractor project specifications take one of two positions: provide a PQR demonstrating soundness at the production DFT, or require removal from the joint zone before any weld arc is struck.

Non-zinc-bearing primers (alkyd, epoxy, vinyl, wash primers): Lower volatilization hazard, but decomposition gases from some formulations can still introduce hydrogen into the weld deposit. On HSLA base metals with carbon equivalents above about 0.40, that hydrogen matters. A macro cross-section of a first-pass weld over non-zinc primer will tell you quickly whether porosity is present. For mild structural steels (A36, A572 Grade 50), the risk is lower but not zero.

What to document on the WPS form

The WPS form — whether Annex M format or a shop-developed form — must capture base metal surface condition when it deviates from bare cleaned steel. Two approaches:

Weld-over approach: "Base metal surface condition: zinc-rich primer, max 2 mil DFT, manufacturer's certified weld-through formulation. See PQR-2025-04 for qualification test results."

Grind-back approach: "Grind base metal to bright bare metal minimum 2 in (50 mm) from joint centerline each side before welding. Inspector hold point: verify bare metal prior to root pass."

The grind-back approach eliminates the coating variable but adds labor. If your production schedule cannot absorb that labor, qualifying over the primer is worth the investment. Either way, leaving the WPS silent on coating condition is not an option.

For shops using a WPS management platform, confirm that your WPS template has a structured field for base metal condition — not just a free-text notes box. A notes field is easy to skip; a required field is enforced. See how digital WPS tools compare to spreadsheet-based systems on this kind of structured data capture.

Running the PQR with coated coupons

When qualifying over primer, the test coupon procedure mirrors standard groove weld qualification with one addition: apply the actual production primer to the coupon at the actual production DFT before welding. Measure the DFT with a calibrated mil gauge on at least three spots per coupon face and record the measurements on the PQR data sheet.

Standard test requirements still apply per the applicable testing clause:

Two tensile specimens, one across the full weld width
Root and face guided-bend specimens per coupon thickness
Visual examination of all faces

Additionally — and this is not always required by D1.1 but is best practice on coated coupons — request macroetch cross-sections of the completed groove. A macro shows porosity clusters, lack-of-fusion at the root, and any inter-run anomalies that bend tests might not catch. If the macro shows elevated porosity, investigate primer type, DFT, and preheat before spending money on tensile and bend testing.

Review PQR tensile and bend test requirements for a full breakdown of what the testing clause requires by thickness range.

AISC certification and third-party audit exposure

Shops holding AISC certification under the Standard for Steel Building Structures, or any program with a written QMS requirement, must address coated base metal in their quality procedures. Third-party auditors — not just AWS CWIs, but AISC, ASME, and owner-representative quality auditors — look for:

A written shop procedure for coated steel that specifies DFT limits, qualifying primer products, and the grind-back distance when removal is required
WPS forms that explicitly reference coating conditions for each procedure used on coated work
PQR records covering the coating condition if weld-over is your approach
Welder and CWI awareness that primer type and film thickness are controlled variables

The most common finding in audit reports is not fabrication defects — it is missing documentation. A WPS that does not mention primer, paired with a PQR run on bare steel, is a non-conformance even if every production weld looks perfect on the radiograph.

See common WPS deficiencies in third-party audits for the full list of documentation gaps that generate corrective action requests.

Connecting coating qualification to your WPS library

If your shop welds coated steel regularly — and almost every structural fab shop does — you need at least one WPS procedure per process (SMAW, GMAW, FCAW) that addresses the coated condition, each backed by a PQR run on coated coupons at the qualifying DFT. Alternatively, a shop-wide grind-back requirement, enforced as an inspector hold point on every job, eliminates the PQR gap at the cost of consistent grinding discipline.

Neither path is complicated. Both require documentation. The shops that fail coating-related audits are the ones that have neither a qualified coating procedure nor a documented grind-back requirement — just an informal understanding that "the welders know to grind it back."

Informal understanding does not survive a third-party audit. A written, signed, version-controlled WPS does.

Rule library based on AWS D1.1:2025; verify against your governing edition.