Polyurethane and polysiloxane topcoats for steel

Why a system needs a topcoat

The topcoat is the top layer of an anti-corrosion system. It protects the underlying epoxy coats from UV and weathering, provides colour, gloss and easy cleaning, and on industrial sites also resistance to splashes of oils and chemicals. Most of the barrier thickness is built up by the primer and the intermediate coat; the topcoat adds its own share and determines how the structure will look years later.

Polyurethanes and polysiloxanes

Parameter Aliphatic (acrylic) polyurethane Polysiloxane
Curing polyol and isocyanate curing agent siloxane binder, often a hybrid with an acrylic or epoxy component; isocyanate-free
Colour and gloss retention good very high
Coat thickness for the materials in this guide usually 50–100 µm 75 to 175 µm, depending on the product
Features sensitive to moisture during application and curing; protective measures needed when working with isocyanates some products allow the system to be reduced to two coats where the manufacturer has proven this; a special application technique over zinc silicate
Price per litre usually lower usually higher

Exterior topcoats are always aliphatic polyurethanes: aromatic ones darken in sunlight and are suitable for primers and interior coats, but not for a weather-resistant top layer.

How to choose a topcoat

Appearance requirements

For bridges, facades, tanks in plain view and facilities in corporate colours, colour and gloss retention matter — this is where polysiloxanes and the best polyurethanes excel. For process structures, a reliable polyurethane within a proven system is usually sufficient.

Number of coats

Several manufacturers offer two-coat systems with a high-build polysiloxane topcoat instead of the three-coat “primer — epoxy intermediate coat — polyurethane” scheme. This saves time and labour, but is acceptable only for the environments and service lives for which the manufacturer has proven such a system.

Application conditions

Check the minimum temperature (for some polyurethanes 0 °C and even −5 °C), the permitted humidity range and the pot life in hot weather. In summer, steel in the sun gets much hotter than the air, and the pot life drops sharply: for one of the polyurethanes in this guide it falls from 1.5 hours at 23 °C to 50 minutes at 40 °C.

Future maintenance

Polyurethanes with an unlimited overcoating interval are easier to refresh during planned maintenance. The extent of preparation of the old topcoat — cleaning and, where necessary, abrading — is given in the technical data sheet.

Temperature and environment

Standard topcoats are designed for service temperatures of around 100–120 °C continuous; hot surfaces need special materials — see “Heat-resistant coatings”. Polyurethane topcoats are intended for atmospheric exposure; they are not used for permanent immersion.

Working with polyurethanes

  • The isocyanate curing agent reacts with moisture: dew, fog and condensation on a fresh coat impair gloss and colour, and high humidity during application can cause bubbles. Keep the curing agent tightly closed in a dry place.
  • Thin only with the thinner specified in the technical data sheet; thinners containing alcohols must not be used with polyurethanes.
  • Yellows, oranges and reds have poorer hiding power: over a contrasting primer an extra coat may be needed.
  • For some products the mixing ratio by volume depends on the colour — in that case part kits are mixed by weight.
  • Working with isocyanates requires ventilation and respiratory protection in accordance with the safety data sheet. Isocyanate-free polysiloxanes make work on operating sites easier but do not remove the general safety measures.
  • Polysiloxane over a zinc silicate primer is applied using the manufacturer’s technique — usually with a thin “mist” first pass; for details see “Zinc-rich primers”.

Common mistakes

  • Applying the topcoat late in the day when dew will form overnight, or just before rain.
  • Too thin a topcoat over a contrasting primer — patchiness and a rapid loss of appearance.
  • One manufacturer’s topcoat over another manufacturer’s epoxy without proven compatibility.
  • Overcoating an epoxy coat after the maximum interval without preparation — delamination of the topcoat.
  • Trying to replace the epoxy intermediate coat with an “extra coat” of polyurethane.
  • Applying material that has thickened in the heat, when the pot life has already expired.

See also: corrosion protection — overview, epoxy primers and intermediate coats, zinc-rich primers.

Materials from leading manufacturers

Polyurethanes

  • Interthane 990 — International (AkzoNobel) — high-gloss aliphatic polyurethane with extended overcoating intervals.
  • Hardtop XP — Jotun — acrylic polyurethane for industrial and marine facilities; a winter grade is available.
  • Acrolon 2330 — Sherwin-Williams — acrylic polyurethane with very high weather resistance.
  • Carbothane 134 HG — Carboline — thin-film high-gloss polyurethane with unlimited overcoating.
  • PPG SIGMADUR 550 — PPG — acrylic polyurethane that cures down to −5 °C.
  • RETANTECT 6000 — Kansai Paint — high-build polyurethane enamel based on an acrylic polyol.
  • Hardtop AX — Jotun — high-gloss acrylic polyurethane topcoat with good chemical resistance.
  • Hardtop HB — Jotun — matt high-build acrylic polyurethane topcoat with very good colour retention.
  • Futura Classic — Jotun — glossy acrylic polyurethane topcoat with low dry spray.
  • TEKNODUR 0050 — Teknos — semi-gloss aliphatic polyurethane topcoat that can be overcoated with itself for up to 18 months.
  • TEKNODUR COMBI 3430-05 — Teknos — high-solids polyurethane paint with anti-corrosive pigments for single-coat application direct to metal.
  • TEKNODUR COMBI 3560-09 — Teknos — polyaspartic coating with up to 93 % volume solids: single-coat protection of metal, application down to −5 °C.
  • Acrolon 218 HS — Sherwin-Williams — fast-drying acrylic polyurethane topcoat for shop application, direct over organic zinc primers.
  • Hi-Solids Polyurethane — Sherwin-Williams — high-solids gloss acrylic polyurethane topcoat, applicable down to −7 °C.
  • PPG SIGMADUR 520 — PPG — high-build semi-gloss acrylic polyurethane finish, unlimited recoat, cures down to −5 °C.
  • Interthane 870 — International (AkzoNobel) — high-build semi-gloss acrylic polyurethane finish, 75–125 µm in one coat.
  • Acrolon C137V2 — Sherwin-Williams — two-pack acrylic urethane gloss finish with long-term colour and gloss retention.
  • Acrolon 7300 — Sherwin-Williams — high-solids acrylic urethane finish in gloss and semi-gloss, approved topcoat for FIRETEX.
  • PPG AMERCOAT 450 H — PPG — gloss aliphatic polyurethane topcoat with unlimited recoatability, applied from −7 °C.
  • PPG SIGMADUR 1800 — PPG — two-pack high-gloss polyurethane topcoat curing down to −5 °C.
  • Interfine 629HS — International (AkzoNobel) — two-pack isocyanate-free high-gloss acrylic finish for epoxy systems.
  • Interfine 691 — International (AkzoNobel) — isocyanate-free high-gloss epoxy acrylic finish, applicable from −5 °C.
  • Futura AS — Jotun — fast-drying gloss aliphatic acrylic polyurethane topcoat for atmospheric exposure.
  • Hardtop Flexi — Jotun — flexible aliphatic acrylic polyurethane gloss topcoat, direct-to-metal capable.
  • PPG SIGMADUR 2800 — PPG — fluoropolymer topcoat with 25 % fluorine, long-term colour and gloss retention.
  • Acrolon 7700 — Sherwin-Williams — high-solids acrylic polyurethane topcoat for C5 systems, accelerator for application from 0 °C.

Polysiloxanes

  • Interfine 979 — International (AkzoNobel) — isocyanate-free acrylic polysiloxane.
  • Carboxane 2000 — Carboline — polysiloxane for two-coat systems.
  • PPG PSX 700 — PPG — high-gloss siloxane, applied over zinc primers.
  • TEKNOSILOX 3351 — Teknos — polysiloxane topcoat with 92 % volume solids for high-durability systems on steel structures, tanks and bridges.
  • PPG PSX 805 — PPG — low-VOC satin engineered siloxane topcoat, applied direct to zinc primers.
  • Hardtop Optima — Jotun — isocyanate-free high-gloss polysiloxane topcoat, recoatable at any stage of curing.
  • PPG PSX ONE 750 — PPG — one-pack high-gloss acrylic polysiloxane topcoat with unlimited recoat window.
  • Interfine 878 — International (AkzoNobel) — isocyanate-free acrylic polysiloxane finish with long-term gloss and colour retention.

Single-pack topcoats

  • Conseal Touch-Up — Jotun — single-pack acrylic touch-up coating that can be applied below freezing.
  • Pilot QD Topcoat — Jotun — single-pack quick-drying alkyd enamel for exterior and interior use.
  • Sher-Cryl M770 — Sherwin-Williams — one-pack water-based acrylic gloss or sheen finish, also a sealer coat over FIRETEX.

This list is not exhaustive. The final choice is made on the basis of the manufacturer’s documents and the project requirements.

Frequently asked questions

Which should I choose: polyurethane or polysiloxane?

If long-term appearance and fewer coats are the priority, consider a polysiloxane. If price, a familiar application technique and a wide range of colours matter more, choose a polyurethane. The decision is made within a specific manufacturer’s system.

How many topcoats are needed?

Usually one; with bright colours or over a contrasting primer, two. The number of coats and their thickness are fixed in the system specification.

Can polyurethane be applied at sub-zero temperatures?

Only materials for which this is explicitly permitted, and only to a dry, ice-free surface above the dew point. The time before overcoating increases noticeably in the cold.

Why did the topcoat lose its gloss after a few days?

A common cause is moisture on the fresh coat: dew, fog or condensation shortly after application. Other causes are an incorrect mixing ratio, an unsuitable thinner, or application to an overheated surface.

We will select a topcoat as part of a complete system for your project — call us or message us on WhatsApp/Telegram: +998 94 094 52 27.