You can paint over an old industrial coating if it still adheres well to the steel, if the new paint is compatible with it and if any lead is known and managed. Three tests decide: the condition of the film (ISO 4628, ASTM D610), adhesion (ASTM D3359 or D4541) and a compatibility test patch. The results dictate the choice between overcoating, spot repair and full removal.
This guide is for maintenance managers and asset owners who must decide whether a steel structure, conveyor gallery, walkway or industrial building frame needs to be stripped, or whether the existing coating can stay. Overcoating an old system costs less than stripping everything, but a poor decision can make the whole system peel a few seasons later. The broader context of maintenance repainting is covered in our article on maintenance painting of steel structures.
Why an overcoat can fail even on paint that looks sound
An overcoat fails when the new coat adds more stress than the old one can carry, or when its solvents attack the existing film. A coating that looks intact may have lost part of its adhesion over time.
Three mechanisms come up most often:
- Shrinkage of the new coat. As it cures, paint shrinks and pulls on the layer below. If the old film adheres poorly, it lifts off with the new coat.
- Solvent attack. A strong-solvent paint applied over an aged alkyd or a thermoplastic coating can soften it, swell it or wrinkle it (lifting).
- Cumulative thickness. Each maintenance cycle adds thickness; beyond what the system tolerates, the film becomes brittle and cracks.
Our article on coating failure modes describes these failures in detail.
Assessing the existing film: ISO 4628 and ASTM D610
The first step is an objective survey of the coating condition, area by area, using recognized assessment standards. Without that survey, the decision rests on an impression.
| Standard | What it assesses | Use in the decision |
|---|---|---|
| ISO 4628-2 | Blistering | Blisters point to lost adhesion or contamination under the film |
| ISO 4628-3 | Degree of rusting | Measures the share of the surface where rust breaks through |
| ISO 4628-4 and -5 | Cracking and flaking | A cracked or flaking film is a poor base for an overcoat |
| ISO 4628-6 | Chalking | A chalky film must be cleaned before any new coat |
| ASTM D610 | Rust grade (visual scale) | North American equivalent for quantifying rust |
| ASTM D714 | Blistering (size and frequency) | Complements D610 in North American specifications |
The survey covers the whole structure, separating exposed areas (edges, connections, column bases, condensation zones) from sheltered ones. It often shows that some areas can be overcoated and others must be stripped.
Measuring adhesion: ASTM D3359 or ASTM D4541
Adhesion of the old film is the criterion that weighs most, because the new coat will never hold better than what lies beneath it. Two families of tests are used.
- ASTM D3359 (tape test). Method A (X-cut) suits thick films; Method B (cross-cut) suits thin films. The ISO equivalent of the cross-cut is ISO 2409, and ISO 16276-2 adapts it to field use. The test is quick and inexpensive, but qualitative.
- ASTM D4541 (pull-off test). A glued dolly is pulled off with a portable tester, which gives a stress value and a failure mode. The ISO equivalent is ISO 4624, and ISO 16276-1 adapts it to field use.
The failure mode matters as much as the value: a break between steel and primer, or between two old coats, marks the weak point of the system. Acceptance criteria are set in the specification, in line with the new paint's data sheet. Tests are spread over representative areas, not only the best-looking ones.
Identifying the old coating type: the ASTM D5402 solvent rub
The solvent rub test per ASTM D5402 helps determine whether the old coating is thermoset or thermoplastic, which governs the choice of new paint. The surface is rubbed with a cloth soaked in a given solvent, and you observe whether the film softens, dissolves or stains the cloth.
- Film that resists: the coating is probably thermoset (epoxy, polyurethane). It generally accepts a wide range of paints after scuff sanding.
- Film that softens or dissolves: the coating is probably thermoplastic (vinyl, chlorinated rubber, some acrylics). A strong-solvent paint may lift it.
- Film that wrinkles slowly: often an aged alkyd or oil-based paint, sensitive to strong solvents.
When the structure's history is available (original specifications, product sheets), it complements the test without replacing it.
The compatibility test patch: the step you do not skip
A test patch is the only reliable way to confirm that a new system will hold on an old one. The practice is described in ASTM D5064, which covers the patch test for assessing coating compatibility.
The typical approach:
- choose representative areas, including degraded and exposed ones;
- prepare the surface exactly as planned for the job (cleaning, scuff sanding, spot repairs);
- apply the full planned system at the data sheet thicknesses;
- let it cure per the data sheet, then inspect for lifting, wrinkling, blistering and cracking;
- run an adhesion test on the patch to check the old and new layers together.
The patch cure time must be built into the schedule: it is often the delay people are tempted to skip.
Overcoat, spot repair or full removal: how to choose
The choice is made from the test results, area by area, and not for the whole structure at once. The table summarizes the logic.
| Condition found | Usual option | Typical preparation |
|---|---|---|
| Adherent film, little rust, compatible | Full overcoat | Washing, scuff sanding, cleaning per SSPC-SP 1 |
| Adherent film, localized rust | Spot repair then overcoat | Power tool cleaning SSPC-SP 3, SP 11 or SP 15 of rusted areas |
| Weak adhesion or incompatibility | Full removal | Abrasive blasting to the grade the new system requires |
| Excessive cumulative thickness or cracked film | Full removal | Abrasive blasting, often with containment |
| Lead present | Depending on condition: encapsulation, overcoat or removal under containment | Method suited to health and safety requirements |
For spot repair preparation, our article on power tool cleaning SSPC-SP 3, SP 11 and SP 15 explains what each grade allows. When full removal is chosen, abrasive blasting prepares the surface for the new system.
Lead changes the decision
Before any preparation, you need to know whether the old paint contains lead or other heavy metals. Structures painted long ago often do; only laboratory analysis of samples confirms it.
Lead has three consequences:
- Health and safety. Any operation that creates dust or chips (sanding, blasting, scraping) requires protective and monitoring measures that meet provincial regulations (CNESST in Quebec, the Ministry of Labour in Ontario).
- Containment and waste. Debris must be contained, characterized and disposed of as hazardous waste where required.
- Strategy. An encapsulating overcoat, which limits disturbance of the film, may be preferable to removal, provided adhesion allows it.
Our lead paint removal service describes the approach when full removal is needed.
What to send for a quote
To price repainting over an existing coating, send overall and close-up photos, dimensions, what you know about the old system, any lead test results and the exposure environment. Our crews and qualified subcontractors run adhesion and compatibility tests on site, from LaSalle, in Montreal, throughout Quebec and Ontario, for example in Quebec City, in Mauricie and Centre-du-Québec and in Ontario. The service is described on the steel structure painting page, and the systems on the protective coatings page. Pricing is on consultation: use the request a quote form for a reply within 24 business hours.
Frequently asked questions
Can epoxy be applied over an old alkyd paint?
It is risky without testing. The solvents and shrinkage of an epoxy can lift an aged alkyd. A test patch per ASTM D5064 and an adhesion test confirm compatibility or not.
Which adhesion test should be used in the field?
The ASTM D3359 tape test gives a quick first indication. The ASTM D4541 pull-off test gives a value and a failure mode, which are more useful when deciding to overcoat a large area.
How long should the test patch be left before concluding?
At least the cure time on the new paint's data sheet, before inspection and adhesion testing. It must be built into the schedule.
Does lead paint always have to be removed?
No. If adhesion is good, an encapsulating overcoat may be an option. The decision depends on film condition, exposure and regulatory requirements.
Hicham M, P Eng, PMP
Engineer and project manager (PMP) at Induscoat. Over 16 years of experience in industrial coatings, composite repairs and wear protection on mining, energy and petrochemical sites in Canada and internationally.