Quick Answer: Surface prep is graded, not binary. SSPC standards run from solvent cleaning through white metal blast cleaning, and each step up removes more contamination, takes more labor, and costs more. The grade your specification names is therefore a cost decision as much as a durability one.
Two bids for the same building can differ substantially and both be honest, because "prepare the surface" is not one thing. It is a graded scale, the grade is where most of the labor goes, and a bid that does not name a grade has not actually told you what it is selling.
The grade is the specification's real cost lever
Coating material is a comparatively predictable line item. Preparation is not: it is labor, equipment, containment and disposal, and it scales with how much of the existing surface has to come off.
That is why the single most useful question about a coating specification is which standard it names. Everything downstream — crew size, schedule, containment, waste handling — follows from that one line.
The ladder, and what each rung actually permits
These are the SSPC standards, now published under AMPP after SSPC and NACE merged in January 2021 and combined their standards programmes later that year. The SSPC and NACE designations remain in use, and they are still what most specifications name.
SP1 — Solvent Cleaning. Removes oil, grease, soluble salts, drawing compounds and similar contaminants by solvent wipe, steam, alkaline or emulsion cleaning. It removes no rust or mill scale at all. Its importance is that it is a prerequisite to every other standard: blasting over an oily surface drives the contamination into the profile rather than removing it.
SP2 — Hand Tool Cleaning. Non-powered tools: scrapers, wire brushes, chipping hammers. Removes loose material only. Tightly adherent mill scale, rust and old coating stay.
SP3 — Power Tool Cleaning. The same objective with grinders, sanders, powered wire brushes and needle guns. Faster and more thorough than SP2, but the acceptance criterion is still "loose material removed" rather than "bare metal."
SP7 — Brush-Off Blast Cleaning. The lightest abrasive grade. Removes visible oil, grease, dust, dirt, loose mill scale, loose rust and loose coating — while tightly adherent mill scale, rust and coating are permitted to remain. The field test is whether the material can be lifted with a dull putty knife.
SP6 — Commercial Blast Cleaning. All visible oil, grease, dust, dirt, mill scale, rust, coating and oxides removed, but staining, streaks and shadows are permitted on up to 33% of each unit area. This is the workhorse grade for a great deal of industrial steel.
SP10 — Near-White Blast Cleaning. At least 95% of each unit area free of visible residue, with staining limited to 5%. Specified where the service environment is aggressive: immersion, chemical exposure, tank interiors.
SP5 — White Metal Blast Cleaning. No visible residue at any point on the surface. The most demanding and slowest grade, reserved for the most severe service.
SP11 — Power Tool Cleaning to Bare Metal. Power tools taken all the way to bare metal, producing a profile. It exists for situations where blasting is impractical — occupied buildings, containment restrictions, spot repairs — and it is slow per square foot by comparison.
SP12 / WJ — Water Jetting. Removes coating, rust and soluble salts using high-pressure water rather than abrasive. Worth understanding clearly: water jetting does not create an anchor profile. It exposes the profile already there. If the steel has never been blasted, water jetting alone will not give a new coating a mechanical key.
The two grades most often confused
SP6 and SP10 sound adjacent and price differently, because the difference is not descriptive but quantitative: staining permitted on 33% of each unit area versus 5%.
Getting from 33% to 5% means more abrasive, more passes, more time in the same square foot, and more spent media to contain and dispose of. When two bids diverge and both look credible, this is one of the first places to look.
What actually drives the number
The grade sets the floor. These push it up, and none of them are visible in a photograph of the building:
- Existing coating condition — sound coating that can be overcoated is a different job from failed coating that must come off in full
- Lead or chromate in the existing paint, which changes containment, worker protection and disposal, often more than the removal itself
- Containment, which on an occupied or environmentally sensitive site can rival the preparation cost
- Access, because swing stage, lift or scaffold time is preparation time as much as application time
- Waste volume, since spent abrasive is a disposal line item that scales directly with the grade
- Ambient conditions, because flash rust between blasting and coating means re-blasting, so humidity and dew point govern how much surface can responsibly be opened in a day
What a specification should say
A prep line that can actually be bid, verified and enforced names four things:
- The standard, by designation — SSPC-SP6/NACE 3, not "sandblast"
- Whether SP1 is included, and where
- The required anchor profile range, with the method of verification
- Who is responsible for containment and for disposal of the removed material
If a bid omits the grade, it is not comparable to one that names it. Ask for it in writing before comparing prices, because you may be comparing SP7 against SP10 and reading the difference as a discount.
Why over-specifying is its own waste
The reflex is to specify the highest grade available. That is not free, and it is often not right.
The grade should be matched to the service environment and to the coating system's own published requirement. A manufacturer's data sheet states the minimum preparation for its warranty; going above it buys nothing that the warranty recognises, while going below it voids the warranty. Interior structural steel in a dry, conditioned warehouse and a tank interior in immersion service are not the same problem, and specifying SP5 for both spends real money on one of them for no return.