A faying surface in a high-strength bolted connection is the contact area between two clamped steel plates through which shear is transmitted by friction; a coated faying surface is that same interface after abrasive blasting and the application, as required by the design documents, of a rust-preventive coating (normally an inorganic zinc-rich silicate or a slip-resistant zinc silicate primer). Its governing design parameter is the slip factor mu, defined as the force that produces slip of the faying surface divided by the sum of the bolt pretension forces acting normal to that surface.
TL;DR - three conclusions
- Do not paint it by default. Clause 6.3.7 of GB 50205-2020 states that, unless the design requires otherwise, faying surfaces shall not be painted, and that oil, loose rust, burrs and weld spatter are equally prohibited.
- Painting is only acceptable when the design states it and the slip factor is taken from a recognised value or a test. Among organic options, only inorganic zinc-rich coatings carry direct tabulated values (JGJ 82-2011: after blasting plus inorganic zinc-rich, mu = 0.35 for Q235 and 0.40 for Q355 and above). For polyurethane zinc-rich or alkyd iron-oxide primers the commentary to GB 50205-2020 reports mean test values of only about 0.2, with a design value of 0.15.
- Once coated, the test must be redone. Specimens must share the material, the process, the surface condition, the batch of fasteners and the storage history of the represented member; they are coated first and tested afterwards, at three specimen sets per process for every 50,000 bolt sets in an inspection lot, and the measured value may not fall below the design value.
1. Key figures
- Slip factors mu (JGJ 82-2011, Table 3.2.4): blasted (shot or grit) 0.45 for Q235 / 0.50 for Q345, Q390 and Q420; blasted then allowed to rust 0.45/0.50; blasted plus inorganic zinc-rich coating 0.35/0.40; wire-brushed loose rust or untreated clean mill surface 0.30/0.35.
- Measured behaviour of coated surfaces (commentary to clause 6.3.2 of GB 50205-2020): polyurethane zinc-rich or alkyd iron-oxide primers give mean values near 0.2, with 0.15 as the reliable design value; zinc-silicate (ZINGA-type) coatings have been shown to support 0.45; slip-resistant zinc silicate paints have a long service record on railway bridges.
- Design shear resistance of one friction-type bolt: Nvb = 0.9 x nf x mu x P, where nf is the number of friction planes (1 for single shear, 2 for double shear) and P is the design pretension force.
- Pretension force P, property class 10.9S: M16 = 100 kN, M20 = 155 kN, M22 = 190 kN, M24 = 225 kN.
- Surface preparation: the blast cleaning grade of a coated faying surface shall meet the design requirement; in practice ISO 8501-1 grade Sa 2 1/2 is specified, with a surface profile Rz typically between 40 and 75 micrometres.
- Film thickness: the minimum coating thickness on a coated faying surface is set by the design. For the surrounding steelwork, where the design does not specify otherwise, GB 55006-2021 clause 7.3.1 requires a total dry film thickness of 150 micrometres outdoors and 125 micrometres indoors, with a tolerance of minus 25 micrometres.
- Inspection lots: one lot covers 50,000 bolt sets (a smaller quantity still counts as one lot); where two or more surface treatments are used, including coated faying surfaces, each treatment is tested separately with three specimen sets per lot (Annex B.0.7 of GB 50205-2020).
- Loading and instrument accuracy: apply 10% of the design slip load first and hold it for 1 minute, then continue at 3 to 5 kN/s until slip; the testing machine shall be accurate within 1% and instrumented bolts, load cells and strain gauges within 2%.
- Application environment: relative humidity should not exceed 85% during blasting and painting, and the steel temperature should be at least 3 degrees Celsius above the dew point.
- Temperature derating: at connection temperatures of 100 to 150 degrees Celsius the resistance of high-strength bolted connections is reduced by 10% (clause 3.1.4 of JGJ 82-2011).
2. Why a painted interface loses friction
2.1 Friction-type and slip-tolerant connections
A friction-type connection transmits shear by clamping force and treats relative slip as its ultimate limit state. A slip-tolerant (bearing-type) connection allows slip and is carried by bolt shear and bearing on the hole wall; it shall not be used where fatigue under repeated load governs. Clause 4.2.1 of JGJ 82-2011 tells the bearing-type designer to keep the contact surface clean or coated as the design requires and not to specify a slip factor at all; for friction-type joints mu is the parameter that decides whether the joint can carry its design shear. That is why two connections in the same frame can carry completely different faying-surface requirements.
2.2 Three ways a film of paint changes mu
- The interface is substituted: what used to be rough steel-on-steel asperity contact becomes a coating-on-coating interface, and slip occurs inside the weaker resin layer.
- The anchor pattern is filled: a blasting profile of Rz 40 to 75 micrometres, once flooded by a self-levelling coating, loses most of its mechanical interlock; the sliding face now reproduces the shape of the coating, not of the steel.
- The coating is crushed and polished: pretension (155 kN for an M20 10.9S bolt) presses the film into the valleys, and repeated loading polishes and crushes it near the holes and plate edges, so the slip load falls over time.
The purpose of coating a faying surface is therefore never routine corrosion cover. It is a deliberate trade of friction capacity for durability, and it has to be justified by tabulated values or by testing on the actual job.
3. The regulatory boundary: unpainted unless the design says otherwise
3.1 The default rule
Clause 6.3.7 of GB 50205-2020 requires faying surfaces to stay dry and clean, free from burrs, sharp fins, weld spatter, weld scars, mill scale and dirt, and states that no paint shall be applied unless the design requires it. "Paint" covers every organic layer in the system, including shop primer, primer, intermediate and finish coats.
3.2 Where the mandatory requirement now sits
Clause 6.3.1 of GB 50205-2020 - which required the fabricator and the erector to carry out slip tests and re-tests separately on faying surfaces, including coated ones, and a separate test for surfaces treated on site - was originally a mandatory provision. Since 1 January 2022 the fully mandatory code GB 55006-2021 has superseded that list of mandatory clauses: the requirement is now carried by clause 7.1.3 of GB 55006-2021, which states that the surface treatment method and blast cleaning grade at high-strength bolted connections shall follow the design documents, and that friction-type faying surfaces shall be slip-tested and re-tested separately after treatment, with results meeting the slip-factor target in the design. The mandatory items 3.1.7, 4.3.1, 6.1.2, 6.2.6, 6.4.5 and 6.4.8 of JGJ 82-2011 were superseded in the same way. In practice: the mandatory code sets the floor, the drawings set the target.
3.3 Coated specimens are coated first and tested afterwards
Clause 6.3.2 of GB 50205-2020 requires the blast cleaning and the minimum coating thickness of a coated faying surface to meet the design. Its commentary adds that metal-sprayed aluminium, galvanising, zinc spraying, organic zinc-rich and other primer systems need a reliable basis for the slip factor, and that only inorganic zinc-rich paint may take values directly from GB 50017. In operational terms: treat, coat, cure and store the specimens together with the members they represent; the specimen must reproduce the real condition; an uncoated specimen or a small trial panel is not an acceptable substitute.
4. Slip factors available to a coated faying surface
| Faying surface condition | mu, Q235 | mu, Q345 and above | Source | Durability and site notes |
|---|---|---|---|---|
| Blasted, uncoated (clean or lightly rusted) | 0.45 | 0.50 | JGJ 82-2011 Table 3.2.4 | Highest friction, no protection; the next process must start within about 4 h and edges are sealed after erection |
| Blasted, then allowed to weep-rust | 0.45 | 0.50 | JGJ 82-2011 Table 3.2.4 | Only valid when the rust layer is uniform; flaking or patchy rust disqualifies the value |
| Blasted plus inorganic zinc-rich coating | 0.35 | 0.40 | JGJ 82-2011 Table 3.2.4 | Adds cathodic protection; control film thickness and full cure; brush application is not permitted for these products |
| Slip-resistant zinc silicate primer | From reliable data or testing; long service record on railway bridges | Commentary to 6.3.2, GB 50205-2020 | A purpose-made product; must be matched to the stripe-coat and sealing strategy | |
| Zinc-silicate (ZINGA-type) rich coating | 0.45 from domestic and foreign test results | Commentary to 6.3.2, GB 50205-2020 | Fine zinc particles; hand brushing shall not be used | |
| Polyurethane zinc-rich / alkyd iron oxide | Mean about 0.2; design value 0.15 | Commentary to 6.3.2, GB 50205-2020 | Large loss of friction; normally excluded from friction-type faying surfaces | |
| Metallised aluminium, hot-dip galvanising | Process and mu both to be established by testing | GB 50017; note to Table 3.2.4 of JGJ 82-2011 | The recurring weak point of duplex systems; never inherit zinc-rich paint data | |
Notes: the values above are code values used for design and lot acceptance. Where plates of two steel grades meet, take the lower value; when rust is removed by wire brushing, the brushing direction must be perpendicular to the direction of load transfer. Any treatment that is not tabulated must be supported by a slip test on the actual job.
5. What a loss of 0.10 in mu costs in bolts
Take an M20, 10.9S connection (P = 155 kN) with two friction planes (nf = 2) in Q355 steel:
- Blasted, uncoated, mu = 0.50: Nvb = 0.9 x 2 x 0.50 x 155 = 139.5 kN per bolt;
- Blasted plus inorganic zinc-rich, mu = 0.40: Nvb = 0.9 x 2 x 0.40 x 155 = 111.6 kN per bolt;
- Accidentally coated with a general-purpose organic primer, mu = 0.15: Nvb = 0.9 x 2 x 0.15 x 155 = 41.9 kN per bolt.
The arithmetic is blunt. Moving from mu = 0.50 to mu = 0.40 removes about 20% of the resistance per bolt, so roughly 25% more bolts are needed for the same design shear (1 / 0.8 - 1), which in turn changes pitch, edge distance, plate size and the clearance needed for the torque wrench. At mu = 0.15 the joint needs about 3.3 times the bolts of the uncoated case (0.50 / 0.15), which usually cannot be fitted at all. A pass of spray over a faying surface is paid for out of the capacity of the joint.
6. Which coatings may go on a faying surface
6.1 Inorganic zinc silicate and slip-resistant zinc silicate
Water-glass or ethyl silicate zinc-rich paints hold a high zinc loading in an inorganic silicate binder, so the film is hard, porous and electrically continuous through zinc-to-zinc and zinc-to-steel contact. It fills the anchor profile far less than a conventional organic film, which is why standards carry tabulated slip factors for it. Rules of use: control the dry film thickness to the design (the minimum thickness of a coated faying surface is a design parameter), allow full cure to a touch-hard, dry state before assembling and testing, and note that the note to Table 3.2.4 of JGJ 82-2011 prohibits hand brushing for slip-resistant zinc silicate and ZINGA-type products. Check the compatibility of the zinc layer with the intermediate and finish coats of the main system, and protect the surface from white rust during wet storage.
6.2 Epoxy zinc-rich, alkyd iron oxide, polyurethane zinc-rich
These are ordinary heavy-duty primers, and on a faying surface they cost friction: the commentary to GB 50205-2020 reports mean values of only about 0.2 for polyurethane zinc-rich and alkyd iron oxide primers. Unless the design recalculates the joint on a measured value, none of them should be sprayed onto the faying area. The typical site failure is exactly the "blast the whole member, then spray the whole member" workflow, where the reserved area is never marked.
6.3 Intermediate coats, topcoats and shop primers
Mica-iron-oxide epoxies and polyurethane or polysiloxane topcoats are levelling, thick barrier layers and have no place inside a faying surface. What happens to an intact shop primer in the joint area must follow the design: retaining it usually reduces mu, so the common practice is local masking during shop blasting or removal during secondary surface preparation.
6.4 When a fire-protection coating is also present
Clause 3.1.4 of JGJ 82-2011 requires that where a member is fire-protected with a coating, the coating thickness at the high-strength bolted connection shall not be less than on the adjacent member. This does not conflict with "do not paint the faying surface": fire protection covers the exposed connection geometry and the member surface, while the load-transmitting interface inside the plate pack stays as the drawing specifies. The two zones must be colour-coded on the detailing drawings.
7. Application and protection window
| Step | What to control | Common practice and typical failures |
|---|---|---|
| Grit or shot blasting | Blast grade to design, in practice Sa 2 1/2; profile Rz 40 to 75 micrometres | Damp or exhausted abrasive undercuts the profile; sea sand is prohibited; soluble salt measured to ISO 8502-9 and kept below 50 mg/m2 as NaCl |
| Painting | Relative humidity not above 85%; steel at least 3 degrees above dew point | Inorganic zinc silicate cures badly in cold, damp conditions, which hits both adhesion and mu |
| Cure and storage | Assemble and test only after the film is fully dry and at design thickness | Inorganic zinc needs a long curing age; testing too early produces misleading figures |
| Edge preparation | Radiused edges of at least 2 mm, three passes of grinding, or an equivalent treatment | Sharp edges pull the film away and become the first rust points as well as stress raisers |
| Site protection | Keep the surface dry and clean; avoid oil, mud, rain and impact | Masking tape over reserved areas and segregated stacking; rusted-back surfaces need re-treatment and a separate test |
| Pre-erection re-test | The erector performs its own slip re-test; site-treated surfaces are sampled separately | Never roll over the shop report: transport and storage have already changed the surface |
8. Testing and acceptance
Annex B.0.7 of GB 50205-2020 defines the procedure. Think of it as five consistencies, one lot with three specimen sets, and one load-slip curve:
- Five consistencies: the specimen shares the steel grade, the production batch, the faying-surface treatment process, the surface condition (including the coating) and the same batch and property class of fastener set as the member it represents, and is stored under the same conditions.
- Specimen geometry: a double-friction-plane, two-bolt splice tension specimen; the side plates are at least half the thickness of the central plate; the net plate section must stay elastic before slip, and each specimen is tested once only.
- Tightening: snug to about 50% of the standard pretension, then final tightening with every bolt measured within 0.95P to 1.05P.
- Loading: apply 10% of the design slip load, hold 1 minute, then load steadily at 3 to 5 kN/s to failure by slip and record the slip load Nv.
- Calculation: mu = Nv / (nf x sum of Pi), with nf = 2 and the measured pretension forces of the bolts on the slipping side; round to two decimals and accept only when the measured value is not less than the design value.
- If the result fails: first check specimen-to-member consistency, pretension and contamination; re-treat the surface and retest. If it fails again, change the process or revise the design; a friction joint is never accepted "close enough".
Two further limits are worth writing into an inspection agreement: test conditions of 10 to 35 degrees Celsius and relative humidity below 80%; and the observation from third-party laboratories that an over-thick or uncured film is the most frequent cause of low results, because slip then happens inside the coating rather than at the steel interface.
9. Interface with the main coating specification
- Mark the zones on the connection drawings: designate each faying surface as "no coating" or "coated faying surface", with the design mu, the product, the film thickness and the boundary of the area (usually offset from the hole centreline), so the shop never sprays the whole assembly by habit.
- Reserve now, seal after tightening: once final tightening has been inspected, stripe-coat or seal the bolt heads, washers, plate perimeters and gaps. This closes the entry points for water and chlorides without disturbing the pretension or the friction path.
- Site-treated surfaces form their own lot: disc-ground or site-blasted surfaces need their own specimens and their own acceptance, never a merged judgement with shop-treated ones.
- Keep two thickness records: one for the faying-surface coating against the design minimum, one for the main system against GB 55006-2021 clause 7.3.1.
Frequently asked questions
Q1. The shop sprays whole members - can the faying surface simply be included?
No. Clause 6.3.7 of GB 50205-2020 prohibits painting unless the design requires it. Once coated, mu has to be re-derived for a coated surface - for most organic systems only 0.15 to 0.2 is supportable - the joint must be recalculated, and in practice no matching test report exists.
Q2. The faying surface has already been painted by mistake. What now?
Check the design first, then choose one of two routes: remove the coating back to the specified preparation grade (usually Sa 2 1/2 to ISO 8501-1), re-treat the surface and test; or treat the area as a coated faying surface, run the slip test on coated specimens and let the designer confirm the capacity. Both routes end in test evidence, not in a visual judgement.
Q3. Why may inorganic zinc-rich take 0.35/0.40 but epoxy zinc-rich may not?
Because of how the film fails and what data the code holds. Inorganic silicate zinc is hard and porous, keeps zinc-to-zinc and zinc-to-steel contact, and has a large body of test data behind the tabulated values. Epoxy zinc-rich forms a continuous, tough organic film in which slip occurs internally, so the codes give no value and the number must come from a project test.
Q4. For a coated faying surface, do you coat the specimen before testing or after?
Before. Specimens are blasted, coated, cured and stored in the same batch as the members, brought to the design film thickness and full cure, then assembled, tightened and loaded. Substituting uncoated specimens or a panel from another batch invalidates the judgement.
Q5. Can galvanised or metal-sprayed aluminium surfaces inherit the zinc-rich paint figures?
No. Both GB 50017 and the note to Table 3.2.4 of JGJ 82-2011 state that any other treatment method must have its process and its slip factor established by testing. For galvanised members in a duplex system, the condition of the zinc layer (white rust, zinc salts) and its changed profile must also be considered.
Q6. After erection, how is the joint protected?
After the final-tightening check has passed, seal the bolt heads, washers, plate edges and gaps: first close the exposed steel and the capillary gaps with a compatible sealing coat, then rebuild the main system to the specified film thickness. On bridges and coastal sheds this detailing step is the real weak point of node durability, and it is the compensation for leaving the faying surface bare.
Q7. Are the slip factor and the nut factor the same thing?
No. The torque coefficient (nut factor K) of a large hexagon set, or the clamp force of a twist-off set, describes the fastener itself; the slip factor describes the plate interface. Both must pass, and normally the fastener re-test is confirmed before the slip result is judged.
References
- GB 55006-2021 General code for steel structures, issued 9 April 2021, effective 1 January 2022 (clauses 4.4.3, 7.1.2, 7.1.3, 7.3.1).
- GB 50205-2020 Standard for acceptance of constructional quality of steel structures, clauses 6.3.1, 6.3.2, 6.3.7 and Annex B.0.4, B.0.7.
- JGJ 82-2011 Technical specification for high strength bolt connections of steel structures, clauses 3.1.3, 3.1.4, 3.2.4, 4.1.1, 4.2.1.
- GB 50017-2017 Standard for design of steel structures - slip factors and connection design.
- ISO 8501-1 (preparation grades), ISO 8502-9 (soluble salts), ISO 8503 (surface profile).
- GB/T 1228, 1229, 1230, 1231 (large hexagon fastener sets) and GB/T 3632 (twist-off type sets).
Last updated: 2026-10-21 | By the technical team of Kexin New Materials (Guangdong) Co., Ltd. All values follow the current edition of the cited codes, the project design documents and the site test reports. For a faying-surface specification, send us the steel grade, the corrosivity category and the design slip factor and we will confirm the system with you.