A paint has two separate preservation problems, and they are solved by two different kinds of active. Spoilage in the can is a problem in the aqueous phase and is handled by water-soluble actives such as CMIT/MIT, BIT or bronopol. Mould and algae on the dried film is a problem in the binder and is handled by lipophilic actives – OIT, DCOIT, IPBC and zinc pyrithione. This article is about the second problem: how to choose a film preservative by exposure condition rather than by spectrum table. Selection inside the can is covered separately in our industrial preservative selection guide; the two decisions are independent and both are necessary.
1. What “dry-film protection” actually means
Once a coating dries, the water is gone and the remaining film is a polymer matrix with pigments, fillers and additives. Mould spores land on the surface, germinate when the surface is wet – rain, condensation, dew – and feed on the binder, on surfactant residues and on organic soil. Algae colonise exterior surfaces in the same way, using light and surface moisture.
For an active to work in this environment it must satisfy three conditions simultaneously: it has to be present in the film after the coating dries, it must not wash out in the first rain, and it must be available at the surface where the spores land. Water-soluble actives fail the first two. A film preservative is therefore defined by low water solubility and by the ability to partition into the binder, not by broad-spectrum activity in a liquid test.
| Mechanism of loss | What happens | Design response |
|---|---|---|
| Leaching | rain, condensation and washing extract the active from the film | choose a low-solubility active; test leach resistance; consider encapsulation for wet service |
| Photodegradation | UV breaks down the active at the surface | light stabilisers; higher dose; accept surface loss and protect the bulk film |
| Chemical consumption | nucleophiles, metal ions and alkaline binders react with the active | compatibility screening; addition point control |
| Migration inward | the active partitions away from the surface into the bulk film | formulation and drying conditions; surface enrichment is difficult to guarantee |
| Volatilisation | loss during baking or during long hot service | check boiling point and binder compatibility for high-bake systems |
2. The four film actives, characterised honestly
| OIT | DCOIT | IPBC | Zinc pyrithione | |
|---|---|---|---|---|
| CAS | 26530-20-1 | 64359-81-5 | 55406-53-6 | 13463-41-7 |
| Water solubility | very low | very low | low | practically insoluble |
| Mould and yeast | strong | strong | very strong | good |
| Algae | weak | strong | weak | good |
| Bacteria | weak in film | moderate | weak | moderate |
| Typical position | cost-effective interior and exterior mould control | immersed, marine and severe-exposure coatings | wood, blue stain, high-humidity interiors | exterior durability, established systems |
| Main caution | strong sensitiser; algae gap | higher cost; stricter market restrictions | cost; colour and odour at high dose | zinc discharge; regional restrictions |
Two points are routinely missed in selection meetings. First, no single film active covers mould and algae equally well at practical doses, which is why exterior coatings are usually protected by two actives rather than one. Second, “stronger” is not a useful ranking: DCOIT is more potent than OIT across a wider spectrum, and it is also more expensive and more tightly regulated. The right question is which exposure the coating will actually meet.
2.1 OIT: the default for atmospheric exposure
OIT gives reliable mould and yeast control in interior and exterior architectural coatings at a cost that most formulations can carry. Its weakness against algae is real and should be designed around rather than dosed around. It is also a strong skin sensitiser, which affects plant handling and restricts its use in consumer-adjacent categories in several markets.
2.2 DCOIT: when the film meets water
The two chlorines on the ring raise potency and extend the spectrum to algae, which is why DCOIT became the standard antifouling active for marine coatings. In architectural coatings it is used where condensation, immersion or severe biological pressure makes OIT inadequate. The trade-off is cost and regulatory scrutiny: DCOIT is under closer assessment in several jurisdictions than OIT, and the permitted product types and dose limits must be checked per market.
2.3 IPBC: mould and blue stain specialist
IPBC is excellent against the fungi that discolour wood and grow in high-humidity interiors, and it is a mainstay of wood protection. It is weak against bacteria and algae, and at higher doses it can affect colour and contribute odour. In coatings it is often paired with an algae-active partner for exterior use.
2.4 Zinc pyrithione: durable but under pressure
Zinc pyrithione has a long track record in exterior coatings and gives useful algae control. Its constraints are environmental rather than technical: zinc discharge to water is regulated in several regions, and the active is restricted or under review in some markets. It is also insoluble, so it must be dispersed rather than dissolved, which affects gloss and colour at higher loadings.
3. Selection by exposure condition
| Exposure | Dominant organism | First choice | Partner if needed | Notes |
|---|---|---|---|---|
| Interior, dry, humidity below 65% RH | none in practice | film active often unnecessary | – | do not add cost and hazard where the risk is low |
| Interior, wet rooms, kitchens, basements | mould, yeast | OIT or IPBC | – | substrate moisture matters more than dose |
| Exterior facade, temperate | mould, algae | OIT | DCOIT or zinc pyrithione for algae | north-facing walls and shaded elevations are the worst case |
| Exterior facade, hot humid climate | mould, algae, heavy soiling | OIT plus an algae-active partner | zinc pyrithione or DCOIT | dose must be validated by exterior exposure, not only by accelerated test |
| Immersed, splash zone, marine | algae, barnacles, bacteria | DCOIT | copper or other co-actives depending on market | leach rate governs service life; test the formulated coating |
| Wood, timber in service | wood-decay fungi, blue stain | IPBC | OIT or a second fungicide | penetration and leach resistance decide performance |
| Plastics, PVC, PU articles | fungi on plasticiser | OIT | – | process temperature and compatibility decide |
| Leather and textiles, tropical shipping | mould | OIT | IPBC in severe cases | test on the finished article, not on a swatch |
4. How much to use, and why the number cannot be copied
Film preservative doses are set by a film challenge test on the actual coating: apply the coating to a substrate, cure it, inoculate with a mixed mould or algae suspension, incubate at high humidity, and score growth against the intended service life. A dose that passes in a matt interior emulsion can fail in a high-PVC exterior paint, because the film’s porosity, binder type and surfactant load all change how much active is available at the surface.
| Variable | Effect on the dose that is actually needed |
|---|---|
| Binder type and PVC | high PVC and porous films expose more surface and lose active faster |
| Film thickness | thin films offer less reservoir and less protection time |
| Surfactant and dispersant load | can bind or solubilise the active and change its distribution |
| Pigment system | zinc and iron pigments can interact with the active |
| Exposure climate | hot humid conditions shorten the effective protection window |
| Substrate moisture | damp substrates drive mould growth from below the film, which no film active can fully stop |
The arithmetic on the dosing side is straightforward once the target is known. Working on active ingredient: a 45% OIT grade contains 0.45 g of active per gram, so 200 ppm active in a 1,000 kg batch needs 200 / 0.45 = 444 g of product. A 30% dispersion would need 667 g for the same target, and a technical solid would need just over 200 g – which is why comparing products on price per kilogram rather than price per kilogram of active is misleading.
5. Regulatory filtering comes first
In every market, the film actives in this article are regulated by product type and by use category, and the permitted list is shorter than the technically suitable list. Two consequences for selection: a formulation that is legal in one country may not be saleable in the next, and a technical-grade concentrate never inherits the authorisation of a customer’s finished product. Practical sequence: fix the destination markets and the use category, obtain the permitted active list and dose limits for each, then choose among what remains, then validate the dose.
Two category exclusions are worth stating plainly because they cause expensive mistakes: isothiazolinones are not permitted in leave-on cosmetics in the European Union and are restricted elsewhere, and a coating that comes into contact with drinking water is governed by a separate approval scheme in most markets. Neither is a technical question and neither is solved by lowering the dose.
6. Common failure patterns
| Complaint | Usual cause | Fix |
|---|---|---|
| Mould appears within one season on an exterior wall | surface condensation and soiling dominate; dose validated only in an accelerated test | exterior exposure testing; consider a surface-active partner and a more hydrophobic binder |
| Algae staining despite a film fungicide | the active has a weak algae spectrum at that dose | add DCOIT, IPBC or zinc pyrithione rather than increasing the mould-active dose |
| Yellowing or discolouration of a white paint | interaction with metal ions, or an active with colour at high loading | switch carrier or active; check driers and zinc pigments |
| Loss of protection after one wet season outdoors | leaching; the active is not retained in the film | test leach resistance; consider encapsulation or a less soluble active |
| Growth inside the can before use | an in-can problem treated with a film active | add an in-can preservative; see our industrial preservative selection guide |
| Skin sensitisation incident in the plant | handling of a sensitiser without closed transfer | engineering controls first; the hazard is intrinsic and permanent |
7. FAQ
Can one active cover both the can and the film?
Not at practical doses. The physics is against it: an active soluble enough to protect the aqueous phase is mobile enough to leach out of the film, and an active retained in the film is too insoluble to protect the liquid. Two actives, two functions.
Is OIT or IPBC better for interior mould?
Both work. IPBC is stronger against the fungi that discolour wood and appear in humid interiors; OIT is usually cheaper and adequate for typical wet-room conditions. The choice often comes down to the substrate and to the regulatory position in the destination market.
Do I need both a mould active and an algae active?
For exterior surfaces in humid climates, usually yes. Mould and algae are different organisms with different susceptibilities, and a single active at a practical dose leaves half the problem exposed.
Does the film active affect the wet paint’s own preservation?
Not meaningfully, and it should not be relied on for it. Film actives are poorly soluble in water and stay largely out of the aqueous phase where in-can spoilage happens.
How do I compare two suppliers of the same active?
Compare price per kilogram of active, not price per kilogram of product; check the carrier, the impurity profile that affects colour and odour, and the documentation package. A 45% solvent-borne grade and a 30% dispersion differ by 50% in delivered active per kilogram.
Conclusion
Film protection is a retention problem before it is a toxicity problem. Choose an active that stays in the binder, match the spectrum to the organism that actually grows on that surface, pair a mould active with an algae active where the exposure demands it, and confirm the dose with a film challenge test on the real formulation. Then filter the result through the regulatory position in each destination market. For the active used in this application see the OIT technical article and the OIT-45 product page; for carrier and addition practice see solvent-borne versus water-borne OIT; for in-can preservation see industrial preservative selection.