Aluminium Body Panels: Why Roping Lines Survive the Paint Bake

A roping line (also called a slip band) is a surface band that forms along the rolling direction of an aluminium sheet after stamping or pre-stretching, caused by grain-orientation differences that make lateral deformation uneven. A brush line is the same family of defect once it becomes visible after painting: bands running along the rolling direction of stamped and painted automotive sheet. The shared feature is that the relief lives in the substrate, not in insufficient film levelling, so no amount of extra film thickness or longer flash-off will remove it.

TL;DR (conclusions first)

• Roping relief is 10-30 um peak-to-valley over 1-3 cm of length (Wang Liang et al., Shandong Nanshan Aluminium, 2020), while primer plus clearcoat typically totals only 70-120 um dry. Substrate defect and coating thickness are in the same order of magnitude, so "levelling it out" does not work physically.
• Judgement is now quantitative: VDA 239-400:2017 Roping Test uses a waviness critical value of Sa=0.8 um; Sa above 0.8 um fails. Mills still run a four-grade visual method (grades 1-2 accept, 3-4 reject).
• On the Chinese standards side, YS/T 1720-2025 (aluminium sheet and strip for body panels, issued 2025-04-10, effective 2025-11-01) lists brush-line grade and roping coefficient among the technical requirements and product qualification items, with test methods referencing GB/T 33227-2016.
• The highest-leverage countermeasure sits at the mill: raising hot-roll finishing coiling temperature from the usual 330-340 C to above 370 C for outer-panel grades clearly reduces roping, followed by controlling the continuous distribution of cube-oriented grains along the rolling direction.
• What the paint shop can do is avoid amplifying it: hold the bake window, protect levelling reserve when going mid-coat-free, and close the loop on DOI and haze (ASTM D5767, ASTM D4039) plus long-wave / short-wave waviness instead of a visual glance.

1. Three kinds of bands: separate the owners first

Roping, brush lines and orange peel are used interchangeably in claim reports, yet they differ in defect subject, when they appear and who owns them. Misclassification sends every corrective action in the wrong direction.

1.1 Roping: a deformation band that shows up after forming

When aluminium sheet is stretched transversely, the fibre texture parallel to the rolling direction cannot accommodate uniform lateral extension, so the surface develops long narrow bands of alternating peaks and valleys 10-30 um deep and 1-3 cm long. The inconvenient property is that the blank looks clean; the bands grow during stamping or pre-stretch, which is why incoming inspection can pass and the pressed part still fail.

A Chinese patent application describing automatic roping detection gives the same phenomenon in other units: depth up to tens of micrometres, band width about 1 mm, length up to about 50 mm, and states explicitly that painting and bake curing cannot cover this defect and can even enlarge it. That sentence is the premise of this article.

1.2 Brush lines: texture-driven, visible only after painting

A brush line is defined as the banding that appears along the rolling direction of automotive aluminium sheet after stamping and painting. Engineering practice requires a grade no worse than 2. The prevailing explanation is the continuous distribution of cube-oriented grains along the rolling direction. Compared with roping, brush lines bind mill texture directly to painted appearance: the same sheet is invisible unpainted and shows bright-and-dark bands once painted and baked.

1.3 Orange peel: wet-film levelling, wavelength 0.1-10 mm

Orange peel is a coating defect: atomised droplets fail to flow out before gelation, producing macro texture in the 0.1-10 mm wavelength band. It differs from substrate relief because it is improvable before cure by changing reducer, film build, flash time or atomisation. Doing the same for roping achieves almost nothing.

2. Key figures for quick reference

• Roping peak-to-valley 10-30 um; band length 1-3 cm; patent wording: depth of several tens of um, band width about 1 mm, length up to about 50 mm.
• Waviness critical value Sa=0.8 um (VDA 239-400:2017 Roping Test); scanning area 37.5 mm x 28 mm; specimens pre-stretched to 10%; profilometer warm-up 30-60 min.
• Hot-rolling finishing coiling temperature: ordinary practice 330-340 C; automotive outer panels should exceed 370 C.
• YS/T 1720-2025: 6016-T4P (0.70-1.50 mm) Rp0.2 = 90-140 MPa, Rm >= 200 MPa, A50mm >= 22%, n(10-20) >= 0.28, r90(8-12) >= 0.65; after bake (T82A) Rm >= 260 MPa, Rp0.2 >= 200 MPa.
• GB/T 33227-2016: textured surface roughness Ra 0.7-1.3 um; thickness tolerance +-0.03-0.11 mm; flatness <=10 mm/m; T4P material should be consumed within 6 months.
• DOI per ASTM D5767: incidence 60 deg to the surface normal, slit width 0.1 mm+-0.02 mm, comb shutter 18 slits; mirror-grade specimen DOI=95.6, intra-laboratory repeatability 0.3 unit, reproducibility 0.5 unit.
• Simulated paint bake 185 C x 20 min; pre-ageing nuclei grow from 2-3 nm along the (001) Al direction to over 10 nm during the bake.
• Stretch-bend levelling is normally kept below 3% elongation; stamping averages about 2% and can locally reach about 15%. Yield-point increase after two weeks at room temperature is 48.0 MPa with 5% pre-strain versus 87.9 MPa with none.
• China consumed more than 800,000 t of automotive aluminium sheet and strip in 2024, body-panel sheet accounting for over 35%, growing by more than 15% per year.
• Product qualification: process-stability verification requires at least three heats and three batches, with Cpk>=1.33 and coefficient of variation Cv<=3%.

3. Why the bake does not hide it and often amplifies it

3.1 The scale mismatch

Typical dry builds are clearcoat 35-60 um, basecoat 10-25 um, primer surfacer 30-40 um, against roping relief of 10-30 um. Wet film does not fill valleys and crests equally: gravity and surface tension make crests thinner and valleys thicker, so curing changes the contrast of the wave rather than removing it.

3.2 The bake is itself a development step

Three things happen inside the same 20-minute window. Bake hardening: 6xxx outer sheet is delivered as T4P and gains its strength in the paint oven, where pre-ageing nuclei of 2-3 nm grow beyond 10 nm along the (001) Al direction and yield strength rises from 90-140 MPa to over 200 MPa. Stress relaxation and springback: the deformation bands left by stamping are redistributed as the part relaxes, which tends to set the banding along the rolling direction. Thermal expansion under constraint: heating, soak and cooling in the fixture add stress that superimposes on existing waviness. The word "amplified" in the patent text is therefore not about the film deteriorating, but about the substrate wave reaching a more stable state.

3.3 Optical amplification

What the eye reports as banding is a light-intensity difference. A high-gloss clearcoat acts as a mirror, and slope changes of a few tens of micrometres redirect reflected light and create bright and dark bands. Three instruments quantify it: DOI (ASTM D5767, image clarity C=(M-m)/(M+m)x100 with M and m the maximum and minimum intensities behind the comb shutter; quantity transfer through polished black-glass working standards, primary standard refractive index nD=1.567 corresponding to 100.0 units and nD=1.527 to 93.6 units, where a 0.01 change in refractive index shifts a 60 deg reading by about 1.6 units); reflection haze (ASTM D4039, Standard Test Method for Reflection Haze of High-Gloss Surfaces, which derives a haze index from two specular gloss measurements on high-gloss non-metallic specimens, usually read together with 20 deg gloss); and waviness decomposition by laser scanning instruments into long wave (roughly 0.6-10 mm, orange peel) and short wave (roughly 0.1-0.6 mm, sanding marks and scratches), with OEM limits defined per body zone. With a band width of about 1 mm, roping falls in the long-wave band together with orange peel, which is why a long-wave reading alone cannot separate a film problem from a sheet problem.

4. Judgement and acceptance

4.1 The mill's four-grade visual method

Standard practice: place the specimen on paper, spread black ink evenly, let it flash off for 10-15 s, then abrade lightly with a sandpaper-faced sponge pad, 2-3 passes in one direction along the length, and count the bands. Grade 1: no bands parallel to the rolling direction. Grade 2: 1-5 bands. Grade 3: more than 5 bands. Grade 4: more than 5 bands with spacing under 3 mm. Grades 1-2 accept, 3-4 reject. The documented weaknesses are operator subjectivity, experience dependence, and the fact that roping is fragmented and discontinuous and is easily confused with abrasion marks.

4.2 The instrumented route: 10% pre-stretch plus Sa

VDA 239-400:2017 Roping Test follows the same logic but is repeatable: the waviness specimen receives the same 10% pre-stretch as the bending specimen, a colour-confocal or optical profilometer scans a 37.5 mm x 28 mm field after 30-60 min warm-up, software filtering produces Sa, and 0.8 um is the acceptance threshold. Two details are commonly missed: without pre-deformation the defect is not yet present, and only specimens taken parallel to the rolling direction develop roping.

4.3 Where Chinese standards place it

YS/T 1720-2025 applies to aluminium sheet and strip for bonnets, doors, fenders and deck lids. Outer panels use 6005A, 6014 and 6016; inner panels use 5754, 5182 and 6016; delivery tempers are mainly T4P and O. It lists brush line, roping and appearance alongside chemistry, dimensional tolerance, mechanical properties, bake hardening, high-speed tension, forming limit, kinetic friction, intergranular corrosion, hemming, microstructure, passivation film weight and oiling, and it points the brush-line grade and roping coefficient test methods to GB/T 33227-2016 and its annexes. GB/T 33227-2016, led by Chinalco, is the dedicated national standard for automotive aluminium sheet, covering 24 alloys from 1XXX to 6XXX with emphasis on the T4P and T6B (simulated bake) tempers, and introducing bake-hardening value, four-grade flange formability and brush-line grading.

5. The mill side: three real control knobs

5.1 Finishing coiling temperature

Roping depends on hot-rolling coiling temperature, cold-reduction and the continuous-annealing temperature. In practice the annealing temperature and reduction are fixed, so the lever is coiling temperature: 330-340 C for ordinary product, above 370 C for outer-panel grades. This is the most cost-effective adjustment available.

5.2 Texture: continuous cube-oriented grains

The accepted explanation of brush lines is the continuous distribution of cube-oriented grains along the rolling direction. The same work found that outer panels with a smaller spread of r8-12 in three directions, and with smaller bend angles, gave better roping results, whereas inner panels showed the opposite trend. Anisotropy is therefore a usable predictor of appearance, not only of formability.

5.3 Chemistry and surface condition

A measured 1.0 mm 6016 automotive sheet contained (weight %) Si 1.12, Fe 0.21, Cu 0.01, Mn 0.07, Mg 0.40, Cr 0.01, Ti 0.02, balance Al. Three practical trade-offs follow: excess Si increases crack initiation sites and reduces hemming; Fe must be held low (typically 0.15-0.25% or below) to limit coarse Fe-rich particles that harm surface quality; and 6xxx outer grades are usually designed with excess Si because it raises both bake hardening and indentation resistance. Textured delivery is specified as Ra 0.7-1.3 um in GB/T 33227-2016, and substrate research shows that texture type matters at least as much as the number: electron-beam texture (EBT) gives better post-paint image clarity on aluminium than random electro-discharge texture (EDT) or mill finish, while also affecting phosphate crystal uniformity together with passivation film weight.

6. The coating side: what it can and cannot do

6.1 Three things that will not work

• Extra film build cannot cover it - the scale mismatch in section 3.1.
• Higher bake temperature will not level it out - precipitation accelerates and hardening rises, but springback and thermal stress grow at the same time; 185 C for 20 min is the usual reference cycle and beyond it the gains reverse.
• Polishing will not remove it - polishing addresses short-wave defects below about 0.1 mm (orange peel, sanding marks), not 1 mm bands 10-30 um deep; it only thins the clearcoat.

6.2 What actually protects appearance reserve

• Design the film stack with reserve. A primer surfacer exists partly to level substrate waviness; a 3C1B mid-coat-free route removes exactly that reserve, so on aluminium-heavy bodies the substrate Sa limit must be tightened. System selection and Sa acceptance should be negotiated together, not in separate reviews.
• Do not create a second defect while hiding the first. Thicker primer lengthens solvent escape paths, and pinholes and boiling develop in 10 um valleys; see the related article on pinholes and solvent entrapment.
• Connect pretreatment to appearance. Aluminium sheet is cleaned and passivated before delivery, leaving a Ti-Zr conversion film. At low or moderate film weight, phosphate crystals are of suitable size and cover the surface; at high film weight the crystals coarsen markedly, coverage drops and phosphating is suppressed - coarse phosphate is a simultaneous risk for appearance and adhesion. Silane routes form a three-dimensional Metal-O-Si-O covalent network and generally leave more margin for film adhesion.
• Close the loop on numbers. DOI (ASTM D5767) plus haze (ASTM D4039) plus long-wave and short-wave waviness, together with substrate Sa, allow sheet problems and film problems to be filed separately. Select specimens per ASTM D3964 and use the terminology of ASTM E284 so that batch-to-batch comparisons stay valid.
• Check uniformity before blaming the substrate. Drift in electrostatic bell parameters produces zone-to-zone film thickness differences that are easily misread as appearance differences; see the article on transfer efficiency and booth air velocity.

6.3 Refinish and spot repair

In aftersales, roping is never repaired - it is either re-engineered by replacing the panel or remains visible after a local repaint. Quote and accept against the appearance of the adjacent body zone rather than a factory standard for a new vehicle, because flash-off, bake and levelling conditions in a body shop are not equivalent to an OEM booth.

7. Production troubleshooting, in order

1. Decide substrate-type or film-type: overlay the banded pattern on the painted part with the pattern revealed after ink-and-abrasion on a bare blank. Coincident bands mean substrate; otherwise work the orange-peel path.
2. Verify the incoming inspection was taken parallel to the rolling direction and included 10% pre-stretch - if either is missing, the Sa value is meaningless.
3. Verify textured Ra is inside 0.7-1.3 um and that the texture type (EDT or EBT) matches the contract; specifying Ra alone loses control.
4. Map forming strain: stretch-bend levelling normally stays below 3%, stamping averages about 2% and can reach 15% locally - high-strain zones are where roping appears.
5. Verify the bake is near 185 C x 20 min; record DOI and Sa together when it deviates, not gloss alone.
6. Confirm whether the stack includes a mid-coat and its dry thickness and pass count; mid-coat-free systems need a tighter Sa ceiling.
7. Audit Ti-Zr passivation film weight and phosphate crystal state (film weight measured by a GB/T 16921-type method).
8. If all of the above is clean, escalate to the mill's coiling-temperature and texture records instead of iterating paint parameters.

8. Frequently asked questions

Q1 How do I tell roping from orange peel on the shop floor?

Ink and abrade a bare blank: roping bands can be counted after pre-stretch and run strictly parallel to the rolling direction; orange peel is already visible in the wet film and follows gun passes, not the rolling direction. The overlay test is the quickest field discriminator.

Q2 Can a thicker primer hide roping?

No. Relief of 10-30 um is of the same order as a 30-40 um primer dry film, and wet film does not fill evenly; extra build mainly adds solvent retention, pinholes and cost.

Q3 Is Sa=0.8 um mandatory?

It is the acceptance threshold of VDA 239-400:2017 Roping Test, a German OEM material test procedure rather than a mandatory national standard. In Chinese projects, YS/T 1720-2025 and GB/T 33227-2016 make roping and brush line test items, while the numeric limit is normally set by the vehicle maker's material specification. Put Sa, visual grade and the sampling and pre-stretch conditions in the same agreement.

Q4 Why does a blank that passed incoming inspection fail after pressing?

Because roping is deformation-induced. Fibre-texture incompatibility is not yet expressed on the flat blank, and appears after stamping or a 10% pre-stretch. Incoming inspection must therefore deform the sample before measuring.

Q5 Would a lower bake temperature reduce the banding?

It reduces precipitation hardening and therefore some of the amplification, but it also sacrifices the bake-hardening increment: 6016-T4P needs to move from 90-140 MPa to above 200 MPa for dent resistance. A practical compromise is a low-bake coating system combined with a tightened substrate Sa limit, evaluated together.

Q6 Must outer panels be textured, and does Ra 0.7-1.3 um hurt appearance?

Texturing mainly serves lubricant retention and friction stability in forming, not optics. It does carry into the painted image, and substrate research shows the pattern type matters more than the number: deterministic EBT pockets behave better after painting than random EDT or mill finish. Specify Ra band and texture type together.

Q7 Visual grading and instrument disagree - which prevails?

The instrument prevails, but keep both. Manual grading is cheap and fast but subjective, and fragmented roping is easily confused with abrasion marks; equipment gives better repeatability and reproducibility. Use visual grading as an in-batch screen and Sa for release, and standardise the preparation (10-15 s ink flash-off, 2-3 one-direction passes) in the work instruction.

Q8 Does a mixed steel-aluminium body change the criteria?

It changes the management, not the physics. Steel and aluminium share the same 170-185 C bake, but aluminium is far more sensitive to springback and banding and steel has no roping at all; galvanic isolation at steel-aluminium contact points competes for the same corrective resources. See the related article on galvanic corrosion design for mixed bodies.

9. Sources

• YS/T 1720-2025, Wrought Aluminium Alloy Sheets and Strips for Body Panels (issued 2025-04-10, effective 2025-11-01; technical committee SAC/TC243; approved by the Ministry of Industry and Information Technology).
• GB/T 33227-2016, Aluminium and aluminium alloy sheets and strips for automobiles.
• Wang Liang, Zhang Zedong, Du Min, Xu Kun, Influence Factors of Roping Line of Aluminium Alloy Automobile Panel, Physical Testing Part A (2020) No. 2, Doc No. 1001-4012(2020)02-0021-04, DOI 10.11973/lhjy-wl202002005.
• Feng Jiani, Chen Jianghua et al., Effects of pre-aging on the microstructure and properties of 6016 alloy before and after paint bake cycle, Journal of Chinese Electron Microscopy Society 2012(6):461-468.
• Liu Xingxing, Tang Jianguo, Zhang Xinming et al., Chinese Journal of Non-Ferrous Metals 2014, 24(5).
• Gao G. et al., Materials 2019, 12(21):3587, DOI 10.3390/ma12213587.
• Gao G., Yan L., Li X., J. Mater. Sci. Chem. Eng. 2024, 12:53-64, DOI 10.4236/msce.2024.127005.
• Bottema J., Lahaye C., Baartman R., Zhuang L., De Smet P., Schoepen F., SAE Technical Paper 981007, DOI 10.4271/981007.
• Chinalco Materials Application Research Institute, review on properties and application technology of aluminium alloys for vehicle lightweighting (brush-line definition, effect of Ti-Zr passivation film weight on phosphating, forming markings of 5XXX sheet).
• Chinese patent application 202210602264, roping detection based on machine vision and feature statistics (background art).
• ASTM D5767, ASTM D4039, ASTM D3964, ASTM E284; VDA 239-400:2017, VDA 238-100:2010, VDA 239-200:2013; ISO 6892-1:2016, ISO 10275:2007, ISO 10113:2006.

About this article

Last updated: 2026-10-31

All figures are taken from published standards, standard commentaries, journal articles and patent documents. Before making an engineering commitment, refer to the official text of each standard. The methods and thresholds cited support communication between comparable suppliers and processes and are not a conformity decision for any specific batch.

Published by Kexin New Materials (Guangdong) Co., Ltd.

Related reading: Orange peel and DOI, Automotive body pretreatment: phosphate, zirconium and phosphate-free routes, Galvanic corrosion in steel-aluminium bodies, Transfer efficiency and booth air velocity, Automotive film thickness control (DFT), Electrocoat primer and throwing power, Paint bake booth control, Dirt inclusion and cleanliness control