Overcoating an Unknown Antifouling System: How to Avoid Detachment
Published by Yacht IQ, Dubai Maritime City — part of the Silver Yachts group of companies.
International Yacht Paint puts it in one sentence:
“Follow overcoating times and immersion times carefully. If you don't, it could result in detachment, blistering or cracking of the antifouling.”
That is the entire risk of this job, from the manufacturer of the paint (International Yacht Paint). Note what it does not say. It does not say you will get poor fouling performance. It says the coating will come off.
A vessel arriving in a UAE yard with no coating records is a routine situation, not an unusual one. Boats change hands, yards change, records do not travel. The cost of getting the next step wrong is not a season of weed — it is a full strip and rebuild of a system you have just paid to apply.
Compatibility matters more than product choice
Most owners approach a repaint as a product decision: which antifouling, how many coats, what does it cost. In an unknown-history job the product is the least consequential variable. What matters is whether the new film can live on the old one.
Two failure modes, both described by manufacturers in their own words.
Overcoating too early traps solvent. Hempel: “the effects of overcoating before the stated minimum time could be wrinkling, bleeding through of the base coat, and solvent entrapment and retention, which may cause loss of adhesion or other defects during the coating system's service life” (Hempel, explanatory notes to product data sheets).
Overcoating too late loses the bond. Hempel again: “the effect of exceeding the recommended overcoating interval is typically reduced adhesion between the two coats. This may result in detachment or peeling, which could be further worsened by exposure to humidity, abrasion, temperature fluctuations and/or further overcoating.”
And a third, less obvious one that is worth understanding because it is routinely mis-diagnosed. The new coat can fail inside the old film rather than at the interface. A coatings-failure investigation put the mechanism this way: aged films “may show no visible signs of weakness and seem to be 'paintable,' but when the weakened film is painted over, the contractive forces (curing stresses) that occur naturally as the new film dries and cures put added tension on the aged and weakened paint film” (KTA-Tator case study — a consultancy investigation, not a manufacturer specification). In that case the failure plane sat within the old coating, and the tape test came back at 0A.
Which means a hull that looks sound can be structurally unable to carry another coat, and no visual inspection will tell you.
The asymmetry nobody expects: soft under hard
Here is the single most useful published document for this problem, and it is a matrix rather than a paragraph. Pettit's antifouling compatibility chart maps the existing paint type against the new one, and the instructions are not symmetrical (Pettit antifouling compatibility chart, May 2024):
| Existing film | New film | Published instruction |
|---|---|---|
| Soft ablative | Hard antifouling | Remove |
| Soft ablative | Durable ablative | Remove |
| Soft ablative | Modified antifouling | Remove |
| Hard antifouling | Durable ablative | Sand and apply |
| Hard antifouling | Soft ablative | Heavy sand and apply |
| Hard antifouling | Modified antifouling | Remove |
Read down the first column and then the second. A soft eroding film under anything is a removal instruction. A hard film under a softer one is a sanding instruction. The old paint has to be able to carry the new one mechanically, and a film designed to wear away cannot.
Pettit adds a specific route for one case: “tin copolymer paints and modified antifouling paints (SPC technology) should be sealed with a coat of Tie Coat Primer, allow to dry 4 hours and apply new antifouling paint.” And a disclaimer worth quoting: “this document is intended as a guide only, additional preparation may be involved due to condition and age of substrate.”
The accumulated-layers problem is real too, though nobody publishes a limit. Seajet: “excessive layers of old antifouling can lead to detachment of the system from the hull.” We looked for a published maximum number of layers or total film build and found none. Anyone quoting you a figure is quoting a rule of thumb.
Identifying an existing system without records
The widely repeated advice is a thumb rub or solvent rub to tell an eroding film from a hard one. We could not find that test in any manufacturer, standards or class document. It is workshop practice, and it is fine as workshop practice, but it is not a specified method and should not be the basis of a system decision.
The manufacturers take a more useful approach: they design the guess out. Hempel's Hard Racing data sheet gives a preparation route keyed explicitly to not knowing:
“Existing old hard matrix antifouling or an unknown antifouling: High pressure fresh water clean, wet abrade, remove dust.”
“If condition of previous antifouling is poor, seal with 1 coat of Hempel's Underwater Primer.”
“If previous antifouling is in general bad condition, it is recommended to remove previous coats and prime before applying antifouling.”
Note that “hard matrix or unknown” are handled identically. That is the manufacturer saying: stop trying to identify it, and treat it as the more demanding case.
Where you do want hard information, there is a proper method for counting what is on the hull. ASTM D4138, Standard Practices for Measurement of Dry Film Thickness of Protective Coating Systems by Destructive, Cross-Sectioning Means, uses a cutting tool — a Tooke gauge — to scribe through to the substrate so the layers can be read under magnification. It applies from roughly 2 to 2,000 µm and “can measure individual coats in multicoat systems or overall coating thickness” (ASTM D4138).
That is the only standardised way to answer “how many systems are on this hull and how thick are they.” On a vessel with no records it is a cheap, fast, and genuinely informative test, and almost nobody asks for it.
One honest limitation of the whole exercise: the accumulated cross-section tells you what is there, not whether it will hold. That is a different question, and it needs a different test.
Test patches, and what they can and cannot prove
No marine antifouling manufacturer we could find publishes a test-patch procedure — no size, no location, no cure time, no assessment criterion. Seajet's only test note is cosmetic: “test an area first. The colour will look different in the water.” That is about colour, and should not be presented as a compatibility test.
What does exist are the adhesion standards, and knowing what each one measures is the point.
ASTM D3359, tape test. Method A, the X-cut, is the one that applies to antifouling, because it covers coatings of 125 µm and above — and accumulated antifouling is always thicker than that. The ratings run 5A (no peeling or removal) down to 0A (removal beyond the X area).
Critically, the standard states its own limit: it “does not distinguish between higher levels of adhesion for which more sophisticated methods of measurement are required.” A tape test screens out gross failure. It does not qualify a system. That is the honest reason a passing tape test is not permission to proceed.
ISO 4624:2023, pull-off. The phrase “multi-coat system” in its scope is why this is the right standard for an unknown stack — it contemplates testing the whole assembly. The result tells you the tensile force at which separation occurs and, more usefully, which plane in the stack is weakest. On an undocumented hull that is precisely the unknown. Note the 2016 edition was withdrawn in June 2023; specify the current one.
The manufacturer's own cautions are worth having in the specification: “Pull off testing is a destructive test and gives information about a localised area,” “once the coating system has fully cured then pull of adhesion tests may be carried out,” and “repair of the area with the full system is necessary in every case” (Hempel, Testing after painting).
We found no published minimum acceptance value in MPa for antifouling-over-antifouling adhesion, from any manufacturer or class society. So a pull-off test on this job is comparative — patch against patch, area against area — not pass/fail against a published number. Design it that way: several patches, sited across the range of condition on the hull, tested after full cure, and judged on where the failure plane sits.
Sealer and tie coats — and the conflict at the centre of them
The sealer coat is the standard answer to an unknown system, and three manufacturers say so:
- International: “Seal incompatible or unknown antifoulings with a suitable primer.”
- Seajet: “If you don't know what antifouling was last used the best solution is to apply a full sealer coat of underwater primer on your boat.”
- Hempel: its Underwater Primer is specified “as a sealer onto old antifouling or as a tiecoat over an epoxy primed surface before antifouling”, at 50 µm, one full coat on previously coated surfaces.
Now the part that gets missed. No manufacturer we could source states that a sealer removes the need to strip in the general case. Every statement is conditional. Sealer for unknown or incompatible but sound. Removal for unsound, excessively built up, or soft-under-hard. Pettit's “Remove” cells for an existing soft ablative are the clearest published statement that a sealer is not an option when a harder film is going on top of an eroding one.
And a sealer is not a universal adaptor. Interlux Primocon — the product most often used for exactly this job — carries hard exclusions: “do not overcoat Primocon with vinyl based antifoulings”, and “do not sand between coats of Primocon or between last coat of primer and first coat of antifouling.”
A sealer can introduce a fresh incompatibility of its own. Check the sealer against the topcoat, not only against the substrate.
The Gulf timing problem
This is where a UAE job differs from the same job in Europe, and it is not about fouling.
A 35 °C yard is at the edge of the application window, not comfortably inside it. International Micron 350 requires product, ambient and substrate temperatures within “minimum 5 °C/41 °F and maximum 35 °C/95 °F”. In a Dubai summer that ceiling is a real constraint on when work can happen, and substrate temperature in direct sun is not the same as air temperature.
Overcoating intervals collapse with heat. From the Micron 350 data sheet:
| Substrate temperature | Minimum overcoat interval | Dry to immersion |
|---|---|---|
| 5 °C | 16 hours | 24 hours |
| 15 °C | 10 hours | 18 hours |
| 23 °C | 6 hours | 10 hours |
| 35 °C | 4 hours | 8 hours |
Hempel's Hard Racing shows the same compression: 8 hours minimum at 10 °C, 4 at 20 °C, 3 at 30 °C. The general rule Hempel publishes for two-pack material is “pot life is halved for every 10°C increase in temperature.”
The operational consequence is that a Gulf schedule is tighter at both ends. There is less margin before the minimum, and materials go off faster in the pot. A programme written for a European yard will not survive translation.
And the out-of-water window is a genuine conflict between manufacturers. International Micron 350 states a maximum of 12 months out of water before immersion. Hempel Hard Racing states “Time to immersion: Minimum: 24 Hours 20°C ; Maximum 6 months.” Hempel's commercial SPC adds the qualifier the others lack: “maximum undocking time depends on the atmospheric conditions (UV radiation, temperature, degree of atmospheric pollution, etc.).”
A factor of two between products, and the more careful of the two makes it conditional on UV and temperature. In a Gulf yard those conditions are the binding constraint, not the calendar. Plan the launch date before the paint date.
When a full strip is the cheaper option
The manufacturers converge on the trigger, and it is condition rather than age. International: “If your existing antifouling is in poor condition, we strongly recommend removing it completely before repainting.” Hempel: “If previous antifouling is in general bad condition, it is recommended to remove previous coats and prime.” Seajet: “When the paint is in poor condition, it should be sanded back to the primer.”
Add Pettit's type-mismatch removals, and add excessive accumulated build, and you have the whole list.
On the cost crossover we will be straight with you: no manufacturer publishes one. The point at which stripping becomes cheaper than sealing is a commercial judgement about this hull, this yard and this schedule — not a specification. Anyone attributing a crossover figure to a paint company is inventing it.
Method matters, and one prohibition is explicit. Seajet: “Never dry sand antifouling.” Wet sanding is the specified route. On chemical stripping, International's caution is that strippers may “soften and affect the GRP surface.”
We deliberately have no cautions to offer on soda blasting, garnet blasting or ultra-high-pressure water jetting of antifouling, because we could not find published ones. We are not going to invent them. What is citable is that flash rust is the water-jetting failure mode, graded under ISO 8501-4, where “grade H is not recommended for overcoating, whereas grades L and M could be acceptable for a primer.”
The waste is hazardous, and in Dubai it is documented
This part is not optional and it is frequently ignored.
The safety data sheet for a common copper antifouling identifies dicopper oxide as a hazardous ingredient, classifies the product “Very toxic to aquatic life with long lasting effects” (H410), and states plainly: “this material and its container must be disposed of as hazardous waste. Dispose of via a licensed waste disposal contractor.”
That SDS covers the liquid product rather than sanding residue specifically — we could not find an SDS clause addressing cured antifouling dust directly, and we are stating the chain rather than overreaching: the film contains dicopper oxide, the material is classified H410 and may meet hazardous waste criteria, therefore the removal residue must be contained and consigned.
In Dubai, the governing instrument is Dubai Law No. (18) of 2024 Regulating Waste Management (full text). Three points matter to a coating job:
- Article 9(a): “No Person may conduct any Waste Management-related activity without first obtaining the relevant permit from the DM.”
- Article 10(a) prohibits storing or treating hazardous waste without a Dubai Municipality permit.
- Article 8 requires producers to maintain registers documenting type, volume and handling method — retained two years for non-hazardous and five years for hazardous waste.
Article 20(a) sets fines of up to AED 500,000, doubled for repeat violations within a year.
An honest scope note: the law defines hazardous waste by reference to a classification determined by a Director General resolution, and we did not locate that resolution — so we cannot confirm that antifouling residue is named on the Dubai list. The prudent reading is that the general regime applies and that a yard should be able to produce its permits and its register. Ask for both before work starts.
Copper and aluminium: the rule, and the exception that catches yards out
The rule, in a manufacturer's words: “Do not use on aluminium or other light-alloy metals. Risk of corrosion in case of direct contact,” and — the sentence that extends it beyond the hull — “copper containing antifouling must not have any electrical contact with aluminium hull and other aluminium components.”
Now the nuance, and it is a real one. Hempel's commercial SPC states that it “may be specified on aluminium hulls provided an efficient anticorrosive system in minimum 2 coats of 150 micron each has been applied,” with the condition that “the anticorrosive system must stay intact during service.”
Same manufacturer, opposite instructions, different products. So the correct question is never “can I use copper antifouling on aluminium” but “does this specific product permit it, behind what barrier, and is the barrier intact?” On a tender, outdrive or aluminium chase boat with an unknown coating history, the barrier's condition is exactly what you do not know.
Document it so the next owner is not in this position
There is no antifouling equivalent of a coating technical file for yachts, so borrow the model from one that exists.
IMO Resolution MSC.215(82) — the Performance Standard for Protective Coatings — requires a Coating Technical File and specifies its contents. Scope caveat, stated plainly: PSPC governs ballast tanks and double-side skin spaces, not antifouling or underwater hull coatings. It is not a requirement for your yacht. It is the best published template for what a coating record should contain (MSC.215(82)).
What it requires recorded is exactly what you will wish you had: product name and identification mark; minimum and maximum dry film thickness; application methods and tools; condition of the surface to be coated; environmental limitations of temperature and humidity; and from the yard, time of coating, thickness, number of layers and ambient conditions. Plus a coating inspector's log and a dated, signed inspection report.
(One accuracy note: PSPC requires a product name and identification mark or number. It does not require a paint batch number — do not attribute that to IMO. Record it anyway; it costs nothing and it is the first thing a manufacturer's technical desk asks for.)
For condition language, ABS publishes a vocabulary that makes a pre-work report defensible rather than subjective — GOOD, FAIR and POOR with numeric definitions — and states the governing rule for this whole article in one line: “unless the original coating system is totally removed, a coating system compatible with the original system should be used in accordance with the paint manufacturer's recommendations” (ABS Guidance Notes).
Note that ABS does not require pull-off testing and specifies no adhesion acceptance value — assessment is visual and photographic. Do not let anyone tell you class demands a pull-off number; it does not.
The decision, in order
- Cross-section the film (ASTM D4138) at several points. Know how many systems and how much total build you are carrying.
- Assess condition against a defined vocabulary, photographed and dated. Sound, or not sound.
- If not sound, or if the existing film is soft-eroding and the new one is harder — strip. No sealer resolves that case.
- If sound but unknown — wet abrade and seal, and check the sealer's own exclusions against your chosen topcoat.
- Test patches before committing the hull, judged comparatively and on where the failure plane sits, not against an invented MPa figure.
- Plan the schedule backwards from the launch date, against the shortest published out-of-water window among the products in the system.
- Confirm the yard's waste permits and register before the first sander runs.
- Write the file. Products, thicknesses, dates, temperatures, humidity, batch numbers, photographs. Keep it aboard.
Steps 1 to 3 cost a fraction of step 3 done twice. Our post on antifouling selection for Gulf conditions covers what to put on once the substrate question is settled.
Stripping, blasting and yard work sit with the yard rather than with us — our group company Silver Yachts covers repair and refit for vessels in the region.
Contact Yacht IQ about overcoating an unknown antifouling system.
Send us photos and any coating records you have and we will advise on a compatible system, the sealer question and the sequence — including what to test before anything is committed. Visit our showroom or get in touch.
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