Prep: why blasting matters
Sandblasting before powder coating removes rust, mill scale, and old coatings and cuts an anchor profile into the metal so the powder can bond mechanically. Without it, coatings tend to fail early by peeling or rusting underneath. Proper Powder Coating Co. in Uintah, Utah (just outside Ogden) blasts and pretreats parts before coating as a standard step, not an upgrade.
The powder is the part everyone sees. The prep is the part the coating depends on and nobody photographs. Sandblasting before powder coating is the single biggest factor in how long the finish lasts, and it is also the step most often shortened when a shop is trying to be the cheapest quote.
This post explains what blasting actually does, what the standards mean, and why we do not skip it.
What blasting does
Blasting does two jobs at once.
First, it cleans. Rust, mill scale, old paint, old powder, weld smoke, and embedded contaminants come off. Chemical cleaning removes oil and grease; it does not remove mill scale or rust.
Second, it profiles. Each piece of media leaves a tiny crater. Millions of them create a surface of peaks and valleys called the anchor profile. Powder melts into that profile during cure and grips it mechanically. A smooth, shiny surface gives the powder almost nothing to hold on to.
SSPC surface prep standards, plainly
The industry uses SSPC (now part of AMPP) standards to describe how clean a blasted surface is. You will see these on job specs, especially for structural, architectural, and government work. Here is what they mean in practice:
| Standard | Name | What it means |
|---|---|---|
| SSPC-SP 5 | White metal blast | All visible rust, scale, paint, and staining removed. Uniform bare grey metal. The cleanest standard. |
| SSPC-SP 10 | Near-white blast | Same as SP 5, but light shadows or streaks allowed on up to 5% of each area. |
| SSPC-SP 6 | Commercial blast | All visible rust, scale, and paint removed, with staining allowed on up to 33% of each area. |
| SSPC-SP 7 | Brush-off blast | Loose rust, scale, and paint removed. Tightly adhered material may remain. Mostly a cleaning and scuffing step. |
For most powder coating on steel, near-white (SP 10) is the practical target, and white metal (SP 5) is used when a spec calls for it. Commercial blast is sometimes acceptable for non-critical parts. Brush-off is typically used on galvanized steel, where the goal is to scuff and clean the zinc, not remove it.
Anchor profile, in mils
Profile is measured as the average distance from peak to valley, usually in mils (thousandths of an inch). For powder coating on steel, a profile of roughly 1.5–3 mils is typical. Check your powder’s data sheet for the exact range.
Too shallow and the bond is weak. Too deep and the peaks can stick up through a thin film, which becomes a rust point. Profile is controlled by media type, media size, blast pressure, and nozzle distance, and it is verified with replica tape or a surface profile gauge.
Blast media
Different media do different jobs. “Sandblasting” is the common name, but actual sand is rarely the right choice.
| Media | Good for | Notes |
|---|---|---|
| Aluminum oxide | Steel, stainless, general prep | Hard, sharp, cuts a clean profile, recyclable |
| Steel grit | Heavy steel, structural, trailer frames | Aggressive, very recyclable, not for aluminum or stainless |
| Garnet | General steel prep, rust removal | Clean, low dust, good profile |
| Glass bead | Light cleaning, aluminum, peening | Leaves a smooth satin surface, less profile |
| Crushed glass | General rust and paint removal | Single-use, low free silica |
Why not sand
Silica sand breaks down into fine crystalline silica dust, which is a serious respiratory hazard, and it is heavily regulated for that reason. It also fractures quickly, leaves more embedded dust, and produces an inconsistent profile. Modern shops use the media above instead.
Mill scale
Hot-rolled steel comes from the mill with a blue-grey oxide skin called mill scale. It looks like a coating. It is not a good one. Mill scale is brittle and loosely bonded, and it expands at a different rate than the steel underneath.
Powder applied over mill scale bonds to the scale. When the scale cracks, from flexing, impact, or a few hundred heat cycles in the sun, the coating lifts off with it, often in sheets. This is the most common reason a coated trailer frame or fence panel starts flaking in large pieces after a year or two. Blasting to near-white removes all of it.
Castings and outgassing
Cast aluminum and cast iron are porous. Air, moisture, and residual oils sit in tiny voids in the metal. When the part heats up in the cure oven, that trapped gas expands and escapes right through the melting powder, leaving pinholes and bubbles.
The fix is an outgassing step: after blasting, the casting is baked at or above cure temperature before coating, so the gas comes out first. Some jobs also use an outgassing-forgiving powder or primer. Hot-dip galvanized steel behaves similarly and gets the same treatment. Skipping this step on castings is how you get a housing that looks like it has a rash.
Powder does not hide anything
Powder follows the surface. A typical single coat is 2–4 mils thick, which is thinner than most weld spatter balls and much thinner than a pit. Anything on the surface before coating will be visible after coating, sometimes more visible, because a uniform color makes texture easier to see.
Before parts come in:
- Knock off weld spatter. Anti-spatter spray helps on the next job.
- Dress welds that will be visible.
- Grind sharp edges slightly; powder pulls thin on razor edges.
- Fill or accept pits. Blasting cleans pits out but does not make them shallower.
This matters most for fabrication shops and off-road and truck accessory builders, where the welds are part of the look.
Pretreatment after blast
Blasting is followed by pretreatment. Freshly blasted steel starts to flash rust quickly, especially in humid air, so it gets washed and pretreated promptly and coated the same day. Phosphate pretreatment improves adhesion and adds corrosion resistance under the film. Aluminum gets a different pretreatment chemistry than steel; the two are not interchangeable.
The order, every time: blast, pretreat, coat, cure. We don’t skip the first two, which is most of the difference. You can see how the process fits together on our sandblasting and powder coating service pages.
If you have parts and are not sure what prep they need, send photos of the actual condition, the material, and the quantity through the quote page. We will tell you what the prep involves and price it along with the coating.