Key takeaways
- Blast welded frames to ISO 8501-1 Sa 2½ before the tanks; alkaline cleaning does not remove MIG silica islands or weld scale, and powder will not bond to either.
- A closed 60 × 60 mm tube heated from 20 °C to 200 °C tries to expand its internal air by about 60%, so every sealed section needs a 6 mm vent at the top and a drain at the lowest point.
- A 2,000 mm steel frame grows about 4.3 mm from 20 °C to 200 °C, so support it at the third points in the oven or it sags and the bolt holes move.
- Powder pulls thin over a weld toe: a 120 µm specification on the face can read 80 µm on the seam, which is why DFT is measured on the weld as well as the flat.
- For C4 service specify 120 µm over a 2–4 g/m² zinc phosphate layer and a 720-hour salt spray coupon to ISO 9227, not a thicker coat over a poor blast.
For pretreatment fabricated frames, the job starts before the tanks, at the blast chamber, and it is not the same job as pretreatment for a flat panel even when both are CRCA. A frame arrives at the line with weld scale, silica islands from the MIG wire, spatter, closed box sections holding air and a geometry that will grow several millimetres in the curing oven. The chemistry is the same seven tanks; the handling, the blast and the hanging are what decide whether the coating survives.
The cost of getting it wrong is not cosmetic. A frame that enters the booth damp blows pinholes along a weld. A sealed tube that is heated without a vent pushes powder off its own seam. A frame supported at two ends in the oven sags, and the bolt holes that were drilled after welding no longer line up with the machine it mounts on.
This guide covers the frame-specific route: blast before the tanks, the seven stages and the numbers to hold on thick sections, the drain and vent rules, oven behaviour and the coverage limits inside closed sections.
What is different about a welded frame?
Three things, and each one has a drawing consequence.
The first is the weld itself. MIG welding on steel leaves a silicate glass island on the bead surface when the wire carries silicon, and that glass is not soluble in an alkaline cleaner. It survives degreasing, it survives phosphating, and it sits under the powder as a non-bonding patch. Blasting removes it; chemistry does not.
The second is geometry. A frame has flanges facing each other, gussets closing corners, and box sections that hold air and water. A flat panel drains in one direction; a frame traps liquid in a dozen places, and the one that matters is the one nobody hung correctly.
The third is mass. A 2,000 mm frame in 60 × 60 × 3 mm sections is around 40–50 kg, and thick steel heats and cools more slowly than 0.8 mm sheet. The oven schedule that cures a panel in 12 minutes may leave a 6 mm gusset under-cured, and an under-cured film passes a thickness check.
What pretreatment fabricated frames need from blast to cure
The table is for a welded frame in 3–10 mm steel, blasted and phosphated on the seven-tank line before powder. The third column is what we will commit to.
| Stage or feature | Typical value for a frame | Ganesh limit or note |
|---|---|---|
| Steel thickness | 3–10 mm plate and sections | 0.8–40 mm across the shop; 10 mm is the press brake limit |
| Blast grade | ISO 8501-1 Sa 2½ | Required on every welded frame; 16 ft × 16 ft chamber |
| Anchor profile | 40–70 µm | Measured on a comparator; a polished weld toe needs the blast most |
| Edge radius from blast | 0.2–0.5 mm | Rounds the laser cut edge and helps powder cover it |
| Degrease | 55–60 °C, 3–5 min | Alkaline; does not remove silica or scale, which is why blast comes first |
| Phosphate coating weight | 2–4 g/m² | Zinc phosphate; 4 g/m² on thick sections, 2 g/m² on 3 mm |
| Final rinse | below 30 µS/cm | Demineralised; salts under the film are the first cause of scribe creep |
| Dry-off | 80–110 °C air | Every fold and tube must be dry, not warm |
| Powder DFT, indoor | 70–80 µm | Measured on the flat and on the weld toe, not only on the flat |
| Powder DFT, C4 outdoor | 120 µm | With blast and 2–4 g/m² phosphate; 150 µm is not an upgrade |
| Cure | 190–200 °C metal temperature | 6 mm sections need longer; oven air temperature is not the record |
| Drain and vent holes | 6 mm minimum | One at the lowest point, one at the highest, per closed section |
| Salt spray | 480 h indoor, 720 h C4 | Coupon to ISO 9227; report creep from the scribe |
| Adhesion | ISO 2409 class 0–1 | Cross-hatch on a production frame per batch |
| Coating growth on a joint | about 0.14 mm per 70 µm face | Mask mating faces or plan the shim stack |
Blasting before the tanks, not after
The blast chamber is the first process step for a welded frame, and it does four jobs at once: it removes weld scale and silica, it removes rust from storage, it rounds the cut edges, and it leaves the 40–70 µm profile that gives the phosphate something to grip.
Doing it after phosphating destroys the conversion coating. Doing it instead of phosphating gives a profiled but unprotected surface that starts to flash rust within hours in a Mumbai monsoon. The order is blast, degrease, phosphate, rinse, dry, powder, and the frame should not sit overnight between blast and pretreatment.
Blast media choice matters on a frame more than on a panel. A coarse grit leaves a deep profile that a 70 µm film cannot fully cover at the peak, so the peaks stand proud and rust first. A 40–70 µm profile with a 70–80 µm film is the workable combination; a 100 µm profile needs 120 µm of powder, and that is a different specification.
The blast also finds the welds that were not cleaned. Spatter that was acceptable under a cosmetic eye becomes a shadowed pocket where the profile is missing, and the powder bridges it rather than covering it. It is cheaper to grind spatter before blasting than to strip and recoat after.
Seven tanks on a thick, folded section
The chemistry does not change with section size, but the dwell and the drainage do.
Degreasing a frame takes the same 3–5 minutes at 55–60 °C as a panel, provided the frame is hung so the cleaner reaches every face. A gusset that closes a corner creates a pocket where the cleaner enters but does not exchange, and the oil that floats out of that pocket recontaminates the face below it. Hanging the frame with the gusset down and the open side up is the difference between a clean corner and a blister three weeks later.
Phosphating runs at 45–55 °C for 3–5 minutes, and the coating weight target moves up slightly on thick sections because the surface area per unit mass is lower and the crystal has more thermal mass to grow on. Two to four grams per square metre covers 3–10 mm work. The measurement is gravimetric on a coupon that travelled with the load, not a colour match.
Rinse and passivation are unchanged: town water rinse, then a demineralised final rinse below about 30 µS/cm with the passivation chemistry at pH 4–5. On a frame, the final rinse is also the stage that finds the blind holes, because a pocket that did not drain in the previous tank releases dilute chemistry into the clean rinse and lifts the conductivity reading. If the conductivity climbs during a frame load, the frame is the cause, not the water.
Dry-off at 80–110 °C has to reach the inside of every fold. A 6 mm flange over a 60 mm return holds a water film by capillary action, and 20 minutes at 90 °C will not always clear it. The check is a white paper towel pressed into the fold on the first frame of the load, not a hand held near the surface. The degreasing guide covers what happens when the oil removal is marginal rather than absent, and the tank-by-tank walkthrough covers the bath control on each stage.
Drain holes, hanging and coverage inside sections
Spray powder is line of sight. On a flat panel that means the whole face; on a frame it means the outside of every section and almost none of the inside.
A 2,000 mm length of 60 × 60 mm tube has about 0.48 m² of internal surface and no way for a gun to reach it. If the design needs the inside coated, the drawing has to give access: a 40 mm hole at each end, or an open section instead of a closed one, or a specification that accepts a bare interior in a sealed, dry cavity.
Drain and vent holes do two jobs. They let the tanks out and the air out. Six millimetres at the lowest point drains a tube in seconds; six millimetres at the highest point stops the oven from pressurising it. Two holes per closed section is the rule, and they belong on the drawing with their positions, because a hole added at the line is a hole in a place that may collect water.
Hanging is the other half. A frame hung from one point swings and shadows; hung from two points at the third of its length, it stays flat and drains symmetrically. Hooks should contact masked or non-visible faces, because every contact point is a bare patch, and a bare patch on a visible face is a touch-up that never matches.
Oven behaviour: expansion, sag and cure
Steel expands about 12 × 10⁻⁶ per °C. A 2,000 mm frame heated from 20 °C to 200 °C grows 12 × 10⁻⁶ × 180 × 2,000 = 4.3 mm. That growth is uniform if the frame heats uniformly, and it returns on cooling, so a frame that is free to move comes out the size it went in.
The problem is restraint and support. A frame resting on two ends in the oven sags under its own weight at 200 °C, where yield strength is roughly 10–15% lower than at room temperature, and the sag does not fully recover. Support long frames at the third points, and never stack frames on top of each other in the oven.
Cure is a metal-temperature event. A 3 mm section reaches 190 °C faster than a 10 mm gusset in the same load, and the oven recorder shows air, not metal. A datalogger on the heaviest part of the load, or a cure schedule that holds the whole load for the thickest section, is the only way to be sure. Under-cured polyester passes a DFT gauge and fails a solvent rub, and the failure appears as chalking or poor adhesion months later.
Salt spray, adhesion and what to specify for the environment
Indoor and sheltered frames take 70–80 µm over 2–4 g/m² of zinc phosphate, which is a C3 system in ISO 12944-5 terms. Outdoor frames, canopy structures and anything within a few kilometres of the coast move to C4: 120 µm, blast to Sa 2½, and a 720-hour neutral salt spray coupon to ISO 9227 with creep reported from the scribe.
The distinction matters because thickness alone does not buy corrosion resistance. A 150 µm film over a poorly blasted surface fails at the interface, and it fails worse than a 120 µm film over a good one because the thick film hides the disbonding until it delaminates in sheets.
Adhesion is checked by cross-hatch to ISO 2409 on a production frame per batch, with the cut made across a weld as well as on the flat. A weld toe is where the film is thinnest and the stress concentration is highest, so a class 0 result on the flat face and a class 3 on the seam is a real outcome and a real finding.
The Ganesh route for welded frames
Frames are cut on the 12 kW fibre laser, formed on the 200 T, 5-axis brake and welded in fixtures on the three CLOOS cells or at the manual stations. They go through the 16 ft × 16 ft blast chamber, then the seven-tank line, then the 18 ft × 14 ft electrostatic powder booth and the curing oven, with a wet paint booth available where the specification calls for a primer or a colour that powder does not cover.
The shot blasting page covers the chamber and the preparation grades, and the paint shop page covers the tanks, the booth and the cure. Film thickness is checked with a DFT gauge and adhesion by cross-hatch on production parts, and the line runs inside an ISO 9001:2015 and ISO 14001:2015 system with an effluent treatment plant for the rinse water.
Frames of this kind sit under the surface treatment and coating hub. The weld sequence that keeps a frame straight before it reaches the blast is covered in the weld distortion guide, and the dimensional consequences of coating on a bolted joint are in the welded assembly tolerances guide.
What to put on a frame drawing
- Section sizes and steel grade, with the thickness of every plate and the weld map.
- Blast and pretreatment requirement, stated as Sa 2½ and a phosphate coating weight range, not as "coated".
- Coating system and DFT, with the environment category to ISO 12944-5 and the durability expectation.
- Drain and vent holes, positioned, with a 6 mm minimum and one pair per closed section.
- Masking schedule: threads, earth studs, machined pads, hinge bores and mating faces.
- Internal coverage decision, stated explicitly as bare sealed cavity or access hole for coating.
- Adhesion and salt spray acceptance, with the standard, the duration and the pass criterion.
- Packing and handling, because a frame stacked on another frame before the film is hard is a frame with marks.
The powder coating cost estimator shows how the blast, the film thickness and the frame area move the cost, which is worth running before the specification is frozen.
When this route is the wrong answer
Blast and powder is the right route for a welded steel frame in ordinary industrial service up to C4. It is wrong in three situations.
The first is a frame that must be conductive at every joint. Powder is an insulator, and a frame with a hundred bolted joints and no masking plan will not carry a bond from one end to the other. If the frame is part of an earthing path, the masking schedule is the specification, and the bond has to be measured rather than assumed.
The second is a frame with a closed cavity that must be coated inside. We run spray powder and wet paint, not dip or electrophoretic coating, so a sealed tube gets no internal film. A design that needs internal coverage needs access holes, or a different process.
The third is a frame that will be welded or drilled on site. Every cut and weld burns the film back and leaves a repair that will not match the oven-cured finish. If site modification is likely, specify touch-up paint with the batch, and accept that the repair is a patch rather than a finish.
Send the frame drawing with the service environment and the masking list through the RFQ route, and the blast grade, the film thickness and the inspection evidence come back with the coating price.
Frequently asked questions
Should a welded frame be blasted before or after pretreatment?
Blast first, then pretreat. Blasting removes weld scale, silica islands and rust and leaves a 40–70 µm anchor profile that the phosphate can form on. If the frame is blasted after phosphating, the blast removes the conversion coating and the powder goes onto bare, profiled steel.
Why do powder-coated frames rust from inside the tube?
Because the inside was never coated. Spray powder is line-of-sight, so the inside of a closed 60 × 60 mm tube stays bare, and condensation forms there first in a humid plant. Either design an access hole for coating or accept that the tube interior relies on a sealed, dry cavity.
How thick should powder be on a fabricated frame?
70–80 µm for indoor and sheltered service over a zinc phosphate layer, and 120 µm for C4 outdoor or coastal service. Above 150 µm the film can crack at weld toes and edges on impact, so thickness is not a substitute for blast quality and pretreatment.
Does a frame distort in the curing oven?
It expands and can sag. Steel grows about 12 × 10⁻⁶ per °C, so a 2,000 mm frame grows roughly 4.3 mm between 20 °C and 200 °C, and it returns on cooling if it is supported evenly. Support it at the third points and never let a long frame rest on two ends in the oven.
Can threads and earth studs be coated?
They can, but they should not be. Powder at 70 µm on an M8 thread will not take a bolt, and a coated earth stud leaves the frame floating. Mask every thread, earth stud, machined pad and hinge bore, and list them on the drawing rather than leaving it to the coater.
Sources and further reading
- ISO 8501-1 — Preparation of steel substrates before application of paints, rust grades and blast cleaning grades including Sa 2½ · reviewed 3 Nov 2026
- ISO 12944-5 — Protective paint systems for steel structures, corrosivity categories and film thickness · reviewed 3 Nov 2026
- ISO 9227 — Corrosion tests in artificial atmospheres, neutral salt spray testing of coated steel · reviewed 3 Nov 2026
- ISO 2409 — Paints and varnishes, cross-cut test for coating adhesion on welded and formed steel · reviewed 3 Nov 2026
Sending a frame for coating?
Send the frame drawing with the weld map, section sizes, service environment and masking list. We will return the blast grade, pretreatment route, film thickness and the evidence per batch.
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