Surface treatment for machine frames that cross an ocean

“Paint blue” is not a specification, and on a frame that will spend thirty days in a container crossing the Pacific it is an expensive omission. Frames arrive with flash rust on machined faces, paint peeling from mill scale that was never removed, and coating on surfaces that were supposed to be bare.

All of it is avoidable with about eighty words on the drawing.

Surface preparation is most of the result

Coating failure is nearly always a preparation failure. Paint applied over mill scale, rust or oil will lift regardless of how good the paint is, because the paint is bonded to a layer that is itself not bonded to anything.

The preparation standard to cite is ISO 8501-1, which defines blast-cleaning grades:

• Sa 1 — light blast. Loose scale and rust removed. Inadequate for anything that matters.
• Sa 2 — thorough. Most scale and rust removed, some staining acceptable.
• Sa 2½ — very thorough. Scale, rust and foreign matter removed; only slight shadows or staining remain. This is the correct default for an export machine frame and what any competent shop expects to see.
• Sa 3 — blast to visually clean steel. Uniform metallic finish. Specified for immersion service and aggressive environments; usually over-buying for a machine frame.

Two more parameters belong alongside the grade. Surface profile — the roughness the blast leaves, typically 50 to 75 μm for standard systems — because too smooth gives the primer nothing to key into and too rough leaves peaks the coating cannot cover. And the time between blasting and priming, which should be stated: blasted steel begins to rust within hours in humid conditions, and a frame blasted on Friday and primed on Monday has to be reblasted. Four hours, or before any visible rerusting, is a normal requirement.

Salt contamination is worth a line if the shop is near the coast, which in Vietnam it usually is. Soluble salts left on the surface draw moisture through the coating by osmosis and cause blistering months later. A maximum of 20 mg/m² chloride, tested per ISO 8502-6, is a reasonable requirement for a frame heading into a marine transit.

A coating system that works

For a machine frame in a normal indoor industrial environment after a sea transit — corrosivity category C3 under ISO 12944, with the transit itself being the aggressive part:

Coat	Product	DFT
Primer	Zinc-rich epoxy or epoxy primer	60–80 μm
Intermediate (optional)	Epoxy build coat	80–100 μm
Topcoat	2K polyurethane	50–60 μm

Total dry film thickness of roughly 120 μm for a two-coat system, 200 μm where the transit or the end environment is harsher. Specify total DFT and require measurement — a coating meter reading is the only way to know what was applied, and the difference between 80 μm and 150 μm is invisible and is the difference between a frame that arrives clean and one that arrives spotted.

Polyurethane topcoats hold colour and gloss; epoxy topcoats chalk under UV but are fine indoors and cheaper. For a machine that will be photographed in a showroom, the polyurethane is worth it.

Machined surfaces: mask, do not paint

This is the failure that causes the most irritation on delivery, and it is entirely a communication problem.

Machined surfaces — rail seats, mounting faces, bores, datum features — must not be painted. Paint on a rail seat has to be removed at assembly, by hand, without damaging the surface you paid to machine flat.

The drawing must say which surfaces are masked, and the masked surfaces need their own protection, because bare machined steel rusts faster than anything else on the part:

• Removable rust preventative — a solvent-based film or a waxy coating that wipes off with solvent at assembly. The normal answer.
• VCI paper or film — vapour corrosion inhibitor, wrapped over the surface and taped down. Excellent for sea freight and cheap. Best combined with the preventative underneath.
• Desiccant inside the wrap where the surface is critical and the transit is long.

State the removal method. An assembly technician who has to scrape a hard protective off a precision surface will damage it.

Threads, bores and edges

Three details that get missed:

Tapped holes should be masked or plugged. Paint in a thread means the bolt either does not start or strips the coating into the joint, and on a bolted structural interface that contaminates the friction surface — which matters, as bolted or welded frame construction explains.

Bores and dowel holes get plugged and protected like machined faces.

Sharp edges hold almost no coating. Paint pulls back from a sharp arris during curing and leaves a thin line that rusts first. Specify edges broken to a 2 mm radius or a chamfer before blasting. It costs a grinding pass and it removes the most common early corrosion site.

Sealed sections and the water problem

A welded box section with an unsealed gap will take on water — during blasting, during washdown, or from condensation cycling in a container. Water inside a closed section corrodes from within, adds unexplained weight, and in cold transit can freeze.

Either seal the sections fully, which means continuous welds where intermittent might otherwise have been acceptable, or provide drain holes at the lowest point in the shipping orientation and say which orientation that is. Half-sealed is the worst of both.

This interacts with the weld specification: if a section must be sealed, the drawing needs continuous welds drawn there deliberately rather than left to the shop’s discretion, as covered in weld symbols on machine frame drawings.

Inspection and evidence

Require three records, all cheap:

1. Blast grade confirmation with a photograph against the ISO 8501-1 comparator, before priming.
2. DFT readings — a stated number of points per square metre, with minimum, maximum and average.
3. Adhesion test on a witness panel prepared and coated alongside the part, tested by cross-hatch or pull-off.

The witness panel matters. Testing adhesion on the frame itself damages the coating; testing on a panel processed at the same time gives the same information without a repair.

The specification to copy

Surface treatment. Break all sharp edges 2 mm minimum before blasting. Abrasive blast to ISO 8501-1 Sa 2½, profile 50–75 μm, soluble salts below 20 mg/m² per ISO 8502-6. Prime within 4 hours with zinc-rich epoxy, 60 μm DFT. Topcoat 2K polyurethane, 60 μm DFT, colour RAL 7035. Total DFT 120 μm minimum. All machined surfaces, bores and tapped holes masked, protected with removable rust preventative and VCI wrap. Drain holes per sheet 3 unless sections are sealed. Supply blast grade photographs, DFT readings and witness panel adhesion test with the part.

That paragraph is the difference between a frame that arrives ready to assemble and one that arrives needing a week of remedial work at the far end.

If you want a frame package checked for coating and masking callouts before it goes out to quote, send it through — the review comes back within 24 business hours. How the coated part then gets crated and shipped is covered in shipping heavy weldments.

See also: powder coating or wet paint on a machine frame and REACH SVHC on painted steel frames.