How AESS steel gets its finish, from the blast to the field touch-up

An AESS finish is decided by two documents, not one. The AESS category on the drawing sets how much grinding, filling and blending the steel gets before any coating, and the coating schedule sets what goes on top: shop primer and paint, a thin-film intumescent where the steel needs a fire rating, hot-dip galvanizing, or zinc with paint over it. The mockup approved before production is what every finished piece gets judged against.

On a structural job the paint bay is where steel goes to be forgotten. On an exposed job it is where the shop finds out whether the grinding upstream was honest, because a blasted surface shows every grind mark, weld ripple and arc strike the mill scale was hiding.

What does the AESS category decide before any coating goes on?

Section 10 of the AISC Code of Standard Practice (ANSI/AISC 303) sorts exposed steel into AESS 1 through AESS 4, plus a custom category, AESS C. The note on what each AESS category asks of a cut and a weld covers the categories themselves. For the finish, the matrix behind them does three things that matter here.

It puts surface preparation to SSPC-SP 6, commercial blast cleaning, in the first category, and every higher category carries what the one below it carries, so any member with an AESS note gets blasted. It removes mill marks at AESS 3, the close-view category. And at AESS 4, the showcase category, it asks for surfaces filled and sanded and for weld show-through minimized: the outline of a weld on the back of a thin plate should not read through the coated face.

Those are fabrication requirements, done with a grinder, filler and abrasive paper before the coating line, and no coating does them afterward. Zinc follows every contour it lands on, and a gloss paint shows flaws a flat one softens.

Why does the blast come before everything else?

SSPC-SP 6 strips mill scale, rust and loose material from the steel and gives the primer a profile to grip. On an exposed member it also works as an inspection, because a blasted surface shows the grinding marks scale was hiding. The American Galvanizers Association notes that grinding followed by commercial blast cleaning keeps grind marks to a minimum, and it is careful to say minimized, not eliminated. Whatever the blast exposes is cheaper to fix before the primer than after.

Two things go wrong here. The first is edges. Dross left on a plasma-cut edge is not steel, and no coating holds to it for long, so exposed cut edges get scraped and ground before the blast, not after. The second is moisture: blasted steel starts to rust as soon as water condenses on it. The dew point rule for blasting in winter applies to exposed work too, only less forgivingly, since a re-blast on a showcase column goes over a surface the grinder already finished.

Paint, intumescent or zinc: what can the schedule call for?

The coating is the architect's choice, written into the specifications and the finish schedule. The shop's job is to prepare for the system named there and nothing else. Four systems turn up on exposed steel, and they ask different things of the bench.

System	What it does	What governs it	What it means on an exposed member
Shop primer and finish paint	A barrier film, primed in the shop and finished in the shop or the field	The coating schedule and the coating maker's data sheet	The film shows the surface under it, so the grinding is the finish
Thin-film intumescent	Swells into an insulating char when heated, slowing the rise in steel temperature during a fire	Approval of the product in standardized fire tests, and the rating the drawings require	A film with real thickness, applied over a primer the coating maker accepts
Hot-dip galvanizing	Zinc bonded to the steel by dipping it in molten zinc at about 450 °C (842 °F)	ASTM A123 for the coating, ASTM A780 for repairs	A grey finish that is not uniform, with vent and drain holes in closed sections
Duplex: zinc plus paint or powder	Galvanizing with a second coating over it	ASTM D6386 to prepare zinc for paint, ASTM D7803 for powder	Color over zinc, with the paint following the texture of the zinc beneath it

An intumescent product has to be approved in standardized fire tests, so the product, its thickness and the primer under it come from the fire design, not from the bench. Galvanizing changes the order of work more than any of the others.

What changes when AESS steel goes to the galvanizer?

The American Galvanizers Association states that the four standard AESS categories do not address all the nuances of hot-dip galvanized coatings. Standard galvanizing accepts rough-looking surface conditions as long as they do not affect corrosion resistance, and an exposed job needs more. The association recommends the custom category, AESS C, with a matrix modeled on the AISC and CISC one, and asks the galvanizer, the fabricator and the erector to settle who answers for each item before fabrication starts.

That modified matrix reads like a shop checklist. Before the kettle: weld spatter off, sharp edges ground, flame-cut edges ground along with their cut faces, low-silicon welding electrodes, and vent and drain holes designed into closed sections, drilled rather than burned, then plugged and smoothed at the higher categories. The association notes that electrodes with more than 0.25 percent silicon can leave a thicker, darker coating over the weld, a raised weld on an otherwise smooth piece, and that grinding before galvanizing does not reliably prevent it. After the kettle: zinc skimmings removed, dross, runs and roughness brought down to projections under 1/16 in, handling marks smoothed, and padded slings so the zinc is not scuffed on the way to the truck. The same list covers protection against wet storage stain, the white deposit that can form on new zinc kept damp without air moving around it, and advises against progressive dipping, where a piece too long for the kettle goes in one end at a time.

Who decides when the finish is good enough?

The mockup does. Under the AESS matrix, visual samples are optional at AESS 2 and required at AESS 3 and AESS 4, and the galvanizers' matrix recommends mockups for its two highest categories. For a finish, the useful sample is a real member, blasted and coated or galvanized the way production will be, and approved before production starts. From then on, an argument about whether a column looks right becomes a comparison with a piece everybody has already signed.

The viewing distance written into the category belongs in that comparison. AESS 2 is meant to be seen from more than 20 ft away and AESS 3 from closer than that, and a finish judged with a flashlight an inch from the steel is being judged against a category nobody specified.

What happens to the finish once the steel leaves the shop?

The finish is only as good as the trip. Padded slings, blocking that does not bear on finished faces, and loads stacked so pieces do not rub are part of the AESS work. Field bolts, field welds and any scuff then need touch-up on site. On painted work that means the same system applied to a prepared spot. On galvanized work, ASTM A780 lists the accepted repairs: zinc-based solder, zinc-rich paint, or metallizing with sprayed zinc.

ASTM A123 also limits what a galvanizer may repair before the steel ships: each bare area 1 in or less in its narrowest dimension, and the total no more than one half of 1 percent of the accessible surface, or 36 sq in per short ton of the piece, whichever is less. A repaired spot can look different from the zinc or paint around it, which is one more reason to put a touch-up on the mockup and look at it from the distance the category names.

What this note does not cover

It does not choose a coating system for a given exposure: that belongs to the architect, the specifier and the coating maker. It does not cover fire-resistance design, since the rating and the intumescent thickness come from the fire design. It does not set film thickness, color or gloss, and it says nothing about a coating warranty. It does not replace the AISC Code of Standard Practice or the galvanizers' published matrix, and both deserve a full read before an AESS job is bid.

Three questions we get about AESS finishes

Is AESS a paint or coating specification?

No. AESS is a fabrication category that sets how exposed steel is prepared and assembled. The coating is a separate line in the specifications and the finish schedule, and a member can be AESS 3 under paint, under an intumescent or under zinc.

Can AESS steel be hot-dip galvanized?

Yes. The American Galvanizers Association recommends specifying the custom category, AESS C, with its modified matrix, because the standard categories do not cover every condition galvanizing creates. Expect a grey finish that is not perfectly uniform, and approve a galvanized mockup before production.

Does every AESS member get blast cleaned?

Yes. Surface preparation to SSPC-SP 6, commercial blast cleaning, sits in the first AESS category, and each higher category builds on the one below it. A member with no AESS note gets whatever surface preparation the contract documents and the coating schedule call for.