Topcoats & Primers Explained
Choosing & Applying International Paints: A Plain English Guide
International (part of AkzoNobel group) is one of the most trusted names in marine coatings .The single most important rule is this: a topcoat is only as good as the primer underneath it. Get the primer/topcoat pairing right and the system will protect your boat for years. Get it wrong and you risk poor adhesion, blistering and premature failure.
Below we break the range into three job-specific systems, cover surface preparation by material, and explain the temperature, thinning and overcoating rules that make the difference between a finish that lasts and one that fails.
First Decision: Above or Below the Waterline?
Almost every coating choice flows from one question — where on the hull is it going? Coatings designed for the topsides and deck are not suitable for permanent immersion, and underwater primers are formulated to work beneath antifouling rather than as a decorative finish.
- Topsides & deck (above the waterline): choose a one-pack gloss enamel such as Toplac (yacht finish) or Interlac (industrial/commercial finish), over the matching primer/undercoat.
- Below the waterline: use a dedicated underwater primer such as Primocon, then an antifouling such as Interspeed.
- Substrate matters too: wood, GRP, steel, aluminium and galvanised steel each need their own preparation — covered further down this guide.
System 1: Toplac Plus over One UP — The Yacht Topside Finish
Topsides & Deck — Above WaterlineToplac Plus is a silicone alkyd premium gloss yacht enamel for use above the waterline on all substrates. It carries UV filters for extra durability, offers excellent flow, and is available in a wide range of bright colours. One UP is its companion single-pack primer/undercoat, designed to work with International one-pack finishes and deck paints on wood, GRP, steel, aluminium and galvanised steel.
Key application figures:
- One UP coats: 3 on steel & aluminium, 2 on GRP, 4 on wood (unthinned). Recommended 48 microns dry per coat.
- Toplac Plus coats: 2–3 coats, sanded with 320–400 grit between undercoat and topcoat. Recommended 26–38 microns dry per coat.
- Thinning: both thin with Thinner No.100 (up to 10%); add an extra 5% to Toplac Plus above 25°C.
- Application: brush or foam roller — Toplac Plus needs no tipping.
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System 2: Interlac over Interprime — The Commercial & Industrial Finish
Topsides & Structures — Above WaterlineInterlac is a single-component alkyd gloss enamel used as a maintenance and cosmetic finish across industrial environments including offshore, petrochemical and chemical plants. Interprime is its alkyd/oleoresinous primer partner — Interlac is only designed to go over an alkyd-based anti-corrosive scheme.
Practical notes:
- Apply over a primed surface only — the primer must be clean, dry and within its overcoating window.
- Coats: by brush or roller, some colours need two coats for uniform cover; aim for 40–50 microns dry per coat.
- Thinner/cleaner: International GTA004.
- Repair areas: prepare breakdown/damage to Sa2½ (or SSPC-SP6 / SP11) and patch-prime before topcoating.
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System 3: Interspeed over Primocon — The Underwater System
Below Waterline — Antifouling SystemPrimocon is a fast-drying, single-pack, aluminium-pigmented vinyl anticorrosive primer for use below the waterline on timber, GRP, steel, aluminium, and cast-iron or lead keels. It also works as a barrier/sealer coat over old, incompatible or unknown antifoulings. Interspeed is the antifouling topcoat that goes over it.
Coats & figures for Primocon:
- Number of coats: GRP – 1; wood/plywood – 3; aluminium, zinc/galvanised steel, lead and steel – 5; barrier/sealer coat – 1.
- Film thickness: 40 microns dry per coat (20 microns minimum is acceptable on GRP).
- Drying: touch dry in around an hour; minimum 3 hours before overcoating with antifouling.
- Thinner/cleaner: Thinner No.3 (thin 10–15% for the first coat on most substrates).
The use of organotin (TBT) antifouling compounds is banned under Irish and EU law. The applicable legislation in Ireland is:
- S.I. No. 82/2008 — Sea Pollution (Control of Anti-Fouling Systems on Ships) Regulations 2008, made under the Sea Pollution Act 1991 (No. 27 of 1991). This implements the IMO International Convention on the Control of Harmful Anti-Fouling Systems (AFS Convention) in Ireland and applies to all ships, including small craft, that are in Irish waters or enter an Irish port or shipyard.
- EU Regulation (EC) No. 782/2003 — directly applicable in Ireland, this banned the application or re-application of TBT compounds on EU-flagged ships from 1 July 2003 and the presence of TBT on any hull from 1 January 2008.
Under Schedule 1 of S.I. 82/2008, all ships must either (1) not bear organotin compounds on their hulls at all, or (2) carry a coating that forms a barrier preventing those compounds from leaching. Applying Primocon as a sealer coat satisfies option 2 — the product is tested and confirmed to prevent TBT leaching when applied to a minimum dry film thickness of 80 microns (at least 2 coats by roller).
Formal certification under the AFS Convention only applies to ships of 400 gross tonnes and above (full International Anti-Fouling System Certificate) and vessels of 24m or more engaged on international voyages (Declaration required). Smaller recreational and fishing vessels below 24m do not require a certificate, but the prohibition on bearing TBT still applies. Penalties for breaches are provided under Section 29 of the Sea Pollution Act 1991.
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Surface Preparation: The 80% of the Job That Matters Most
Preparation by substrate (always clean thoroughly and allow to dry completely afterwards):
- GRP / gelcoat: degrease, then sand with 180–220 grit. Aged or weathered gelcoats are porous — seal with 3 coats of Interprotect (overcoated with One UP) before a topcoat to reduce the risk of blistering.
- Bare wood: sand smooth with 80–180 grit then 280 grit; remove all sanding dust by brushing, dusting and wiping.
- Steel: degrease, then gritblast to Sa2½ (near-white metal). If blasting is not possible, grind with 24–36 grit discs to a clean, bright surface with a 50–75 micron anchor pattern.
- Aluminium: degrease, sand with 24–120 grit aluminium-compatible paper (coarser grades aid adhesion), and prime as soon as possible — ideally within 8 hours.
- Zinc / galvanised steel: degrease, then sand with 60–120 grit.
- Previously painted, good condition: wash with detergent, rinse, dry, then abrade (220–280 grit for One UP; 280–320 for a Toplac Plus topside).
- Previously painted, poor condition: remove all old coatings and start from bare substrate.
Getting the Conditions Right
- Temperature: ambient, substrate and product should all sit between 5°C and 35°C. Toplac Plus prefers a product temperature of 15–35°C. Never apply below 5°C.
- Humidity & dew point: apply only in dry conditions. For underwater priming, keep the substrate at least 3°C above the dew point.
- Thinning: One UP and Toplac Plus use Thinner No.100 (up to 10%); Primocon uses Thinner No.3 (10–15% on the first coat); Interlac uses International GTA004.
- Coats & coverage: follow the per-system figures above — an extra coat increases substrate protection, and foam rollers may need an additional coat to reach full film build.
Safety, Storage & Disposal
These are solvent-based products. Always read the label safety section and the Safety Data Sheet (SDS) before use, and work in a dry, well-ventilated area away from sources of heat and ignition. Wear appropriate protective equipment.
Storage: keep containers firmly closed between uses, out of direct sunlight, and stored between 5°C and 35°C to realise the full shelf life. Most of these one-pack products carry a typical shelf life of around 2 years.