Steel Surface Preparation Standards: SSPC/AMPP SP Grades Explained

Surface preparation is specified to a numbered standard, not to "clean as possible". In North America those standards are the SP grades published by AMPP (the Association for Materials Protection and Performance, formed when SSPC and NACE merged), which is why you see names like SSPC-SP 10/NACE No. 2 — one joint standard carrying both lineages. Preparation is the step most responsible for how long a coating lasts, and the grade is how a specification says what "prepared" means.

The SP grades at a glance

Titles and designations from AMPP's standards listing; the cleanliness wording is summarized from the standards as published by coating manufacturers and consultants, because the standards themselves are sold by AMPP. AMPP's store lists SP 6 as the 2007 edition, status active (reaffirmed). Always specify from the current edition. SSPC-SP 14/NACE No. 8, Industrial Blast Cleaning, sits between SP 7 and SP 6, and wet abrasive blast and waterjet cleaning have their own joint standards such as SSPC-SP 5 (WAB)/NACE WAB-1 and SSPC-SP WJ-1/NACE WJ-1.

Reading the three main blast grades

The blast grades differ by how much staining may remain. Staining is the light shadowing, slight streaking or minor discoloration left on a surface that is otherwise free of visible contamination, and the allowance is measured per unit area of surface:

• SP 6, commercial — up to 33% of each unit area.
• SP 10, near-white — up to 5%.
• SP 5, white metal — none.

Each step up takes more passes, more abrasive and more labor, which is why a specification only asks for as much as the service needs. The ISO 8501-1 grades used in much of the world are commonly cited as approximate equivalents — SP 7 with Sa 1, SP 6 with Sa 2, SP 10 with Sa 2½ and SP 5 with Sa 3 — but they are working approximations rather than identical standards, so a specification should name one system and stay in it.

Surface profile is separate from the grade

The SP grade is about cleanliness. The surface profile — the roughness or anchor pattern the blast cuts into the steel — is specified on its own, normally by the coating manufacturer, because the primer has to key into it. Two surfaces can both meet SP 10 and have different profiles.

Profile is measured with ASTM D4417, whose three field methods are a visual comparator (Method A), a depth micrometer (Method B) and replica tape (Method C); newer editions add a peak-count method, so check the current edition. The pipeline industry shows the separation clearly: ISO 21809-2 prepares its fusion-bonded-epoxy test specimens by blasting with steel grit to ISO 8501-1 grade Sa 2½ and to a peak-to-trough height of 50–100 µm (about 2–4 mils) measured to ISO 8503-4 or -5 — two requirements, stated separately. Our sandblasting service page covers how abrasive choice shapes the profile.

Some systems ask for more than a standard grade. Thermal-sprayed zinc or aluminum, for example, needs an angular profile with a greater amplitude than other coatings, and guidance for it asks for a clean surface ideally to ISO 8501-1 Sa 3, with Sa 2½ satisfactory for zinc (SCI Guidance Note 8.04).

Cleanliness you cannot see

A surface can pass a visual grade and still carry contamination the eye does not register. Solvent cleaning does not remove chlorides or sulfates, and ISO 21809-1 treats surface dust and pre-treatment and salt contamination as their own clauses, separate from blast cleanliness and roughness.

The FHWA study Methodology for Analysis of Soluble Salts From Steel Substrates (FHWA-HRT-14-026) puts the stake plainly: coatings applied over excessive amounts of adverse soluble salts do not provide the expected service life, and the salts cannot easily be removed by washing or abrasive blasting. They can also sit in pits and crevices, under rust. On steel that has been in service — especially coastal or de-iced — salt testing belongs in the inspection plan alongside the grade.

Checking the work

Inspection has three separate questions, each with its own tool:

1. Is it clean enough? The written SP standard governs. SSPC-VIS 1 supplies reference photographs of steel before and after dry abrasive blast cleaning to help judge borderline areas; the guide itself states that the photographs supplement the written standards and are not a substitute for them.
2. Is the profile right? ASTM D4417, to the range the coating data sheet calls for.
3. Is the surface free of dust and salts? Dust and soluble-salt testing, which a visual grade does not cover.

How the grade affects price

Grade is one of the main drivers of a blasting quote, because a higher grade means more material removed and more abrasive, labor and time — plus containment and disposal, which depend on the site and the material removed. The coating manufacturer's data sheet sets the minimum grade, so choosing a lower one to save money puts the coating's warranty and service life at risk.

For the numbers — what blasting costs per square foot by surface, grade and field versus shop — see sandblasting cost. To compare bids from contractors who work to these grades, see our industrial coating contractors directory. For how hot dip galvanizing prepares steel by chemical cleaning — degreasing and pickling — instead, see the hot dip galvanizing guide.

Sources: AMPP surface preparation standards committee for titles and designations; ASTM D4417; ISO 21809-1:2018 and ISO 21809-2:2014 for the pipeline examples; FHWA-HRT-14-026. Checked October 2026.