STP ShaliProtek 2AC PU Surfacer
The two-component aliphatic PU mid-coat that builds tough, UV-stable film between primer and topcoat on steel and concrete.
What is it?
A two-component, aliphatic acrylic polyurethane mid-coat that goes on after the primer and before the finish coat, building film thickness and giving the topcoat a uniform, well-keyed surface to bond to.
Where is it used?
Steel and concrete structures in industrial, coastal, marine and infrastructure settings — offshore platforms, chemical and power plants, bridges, rail stock and heritage structures under a multi-coat PU system.
Why use it?
It closes pinholes and surface irregularities left by the primer, adds impact and abrasion resistance in its own right, and gives the topcoat a smooth, consistent base so the finished system performs for years, not months.
How is it applied?
Mixed 9:1 by volume (Component A:B), then brushed, rollered or sprayed over a primed and dried surface to 50–100 micron DFT, with the topcoat following inside the recoat window.
About STP ShaliProtek 2AC PU Surfacer
On any serious steel or concrete protective coating job, the mid-coat is the layer that quietly decides whether the whole system holds up. STP ShaliProtek 2AC PU Surfacer sits between the primer and the topcoat — a two-component, aliphatic acrylic polyurethane formulated to build film, level out the substrate, and give the final coat a uniform surface to bond to. It’s a liquid, matt-to-egg-shell coating available clear or in shade, and because it’s aliphatic rather than the cheaper aromatic PU chemistry, it holds its properties and doesn’t chalk or discolour the way lesser mid-coats can when a phased application leaves it exposed longer than planned.
Contractors reach for it on work where a coating failure is expensive to fix — offshore structures, petrochemical and pharmaceutical plants, power and gas installations, marine jetties, rail coach exteriors, cement and steel plants, and heritage structures where surface finish matters as much as protection. It resists mild chemicals and solvents, takes real mechanical abuse, and bonds hard enough to keep the topcoat from lifting at the interface. Applied direct to metal or over STP’s ShaliPrime primers on concrete, it’s built to make everything above and below it work as one system, not three separate coats.
The technical insight
The chemistry is what separates a surfacer from an ordinary paint coat. Aliphatic polyurethanes cure through an isocyanate-polyol reaction into a dense, cross-linked film — the aliphatic backbone, unlike an aromatic one, is what gives it UV stability, since aromatic PUs contain ring structures that absorb UV energy and break down, yellowing within months outdoors. Mixed 9:1 by volume, the two components react over a 3–4 hour pot life at 30°C, reaching 52 ± 2% volume solids and curing to a film with 60 ± 5 Shore A hardness after seven days. Because it’s applied at 50–100 micron DFT — thicker than a typical intermediate coat — it fills the surface profile left by blasting or grinding and levels out primer texture, giving the topcoat a consistent base to key into. That’s what keeps a multi-coat PU system from delaminating at its weakest point: the interface between coats.
Where it is used
- Intermediate mid-coat in multi-layer PU protective coating systems on steel and concrete
- Protection of heritage structures and statues
- Coastal and chemical-exposure environments
- Power, oil and gas plants, and offshore structures
- Petrochemical, pharmaceutical and chemical plant structures
- Cement and steel plant structures
- Infrastructure projects such as bridges and industrial buildings
- Marine structures, rail coach and chemical wagon exteriors
Key advantages
- Aliphatic PU chemistry resists UV and weathering without yellowing or chalking
- High impact resistance — 150 kg-inch per ASTM D 2794
- Low abrasion loss (under 80 mg per ASTM D 4060) for high-wear zones
- Strong adhesion pull-off (4–5 MPa) that keeps the topcoat from lifting
- Can be applied direct to metal, or over ShaliPrime primers on concrete
- Flexible enough to move with the substrate without cracking
- Passes 1,000-hour QUV exposure testing per ASTM G 154
- Available transparent or in pigmented shades for finish flexibility
Limitations & considerations
- Short pot life at 30°C (3–4 hours) leaves little room for slow mixed-batch application in hot weather
- Recoat window is bounded (8–48 hours) — miss it and the surface may need abrading before topcoating
- Full cure takes 7 days, so the system shouldn’t be put into aggressive service before then
- Needs correctly prepared, primed substrate — Sa 2/2.5 blast on steel, 28-day-cured concrete under 5% moisture — or adhesion suffers later
- Solvent-borne 2K PU, so standard ventilation and PPE precautions apply during mixing and application
- Theoretical coverage of 8–10 m²/L varies with surface profile, so actual consumption depends on site condition, not the datasheet figure alone
Selection & best practice
Get the substrate right first. On concrete, that means at least 28 days’ curing, moisture below 5%, mechanical grinding or blasting to remove laitance and old coating, and a slightly textured profile — then a coat of ShaliPrime PUM, dried before the surfacer goes on. On steel, blast to Sa 2 or Sa 2.5 depending on corrosion, and prime with ShaliPrime ERO or ShaliPrime Zn Ph, though direct-to-metal is an option where the spec allows it. When mixing, stir Component A alone with a spiral-blade drill mixer at 300–400 rpm until homogeneous, then add the full pre-weighed Component B at its 9:1 ratio and mix again, waiting five minutes for entrapped air to escape before applying. Work within the 3–4 hour pot life at 30°C — in hot conditions, mix smaller batches more often rather than stretching one. Apply by brush, roller or spray to 50–100 micron DFT, thinning 2–5% for brush and roller or up to 15% for conventional spray, and respect the recoat window: 8 hours minimum, 48 hours maximum, or inter-coat adhesion can suffer. Clean tools immediately with STP Thinner — once this starts to cure, it won’t come off.
Specifications
| Chemistry | Two-component aliphatic acrylic polyurethane |
| Finish | Matt to egg-shell |
| Mixing ratio (A:B, by volume) | 9:1 |
| Volume solids | 52 ± 2% |
| Pot life @ 30°C | 3–4 hours |
| Recoat window | 8–48 hours |
| Recommended DFT | 50–100 micron |
| Theoretical coverage | 8–10 m²/L |
| Full cure | 7 days at 30°C |
Industry use cases
Offshore & marine structures
Mid-coat on jetties, platforms and marine steelwork exposed to salt spray and constant UV, where inter-coat adhesion under the PU topcoat is critical.
Petrochemical & pharmaceutical plants
Builds film and chemical resistance on tankage, structural steel and pipe supports where mild chemical splash is routine.
Power, oil & gas infrastructure
Used across structural steel and concrete as the mid-coat beneath a UV-stable PU topcoat on plant and pipeline structures.
Heritage structures & statues
Chosen where finish and long-term colour stability matter as much as protection, since the aliphatic chemistry resists yellowing on visible work.
Rail & transport
Coats rail coach and chemical wagon exteriors that need an impact- and abrasion-resistant film under the topcoat.
Frequently asked questions
Is ShaliProtek 2AC PU Surfacer a primer or a topcoat?
What’s the coverage rate, and how much should I order?
How long before I can apply the topcoat over this?
Can this go directly onto bare steel without a primer?
How tough is the cured film — will it take foot traffic or minor knocks?
Does it need special ventilation or safety precautions on site?
How we position STP ShaliProtek 2AC PU Surfacer
ShaliProtek 2AC PU Surfacer is one layer of a system, not a standalone fix — pair it with STP ShaliPrime PUM or ShaliPrime ERO/Zn Ph as the primer, and STP ShaliProtek 2AC PU TC as the UV-stable topcoat that finishes the job (ShaliProtek 2AC PU TC 50 where added solar reflectivity is needed). Where a spec calls for a pigmented, single-system aliphatic PU coating instead of STP’s three-layer build, Space Arc’s Fosroc range offers Dekguard PU as a comparable option. For waterproofing rather than protective coating — decks, roofs, tanks — our Fosroc Nitoproof range and STP’s ShaliProtek PuR line fit better.
Fosroc Fosroc Dekguard PU — Dekguard PU is a two-component aliphatic PU protective topcoat, not a mid-coat surfacer, but it’s the closest Fosroc equivalent where a spec wants a single-system PU coating instead of STP’s layered primer-surfacer-topcoat build.
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