Intumescent coating relies on a suitable connection between the structural steel, primer and reactive layer. Surface preparation creates the condition required for that system to be applied. It is not a cosmetic step and it cannot be reduced to making the steel look clean. The correct method depends on the substrate, contamination, corrosion, existing coating and selected product.
Key points
- Survey the existing condition before choosing a preparation method.
- Remove contamination and unsound material to the specified standard.
- Treat cleanliness, surface profile and primer compatibility as separate requirements.
- Protect prepared steel from recontamination before coating.
Begin with a condition survey
Newly primed steel, weathered primer and previously coated existing steel present different risks. The survey should identify oil, grease, dust, rust, mill scale, salts, loose material, mechanical damage and areas of unknown coating.
Representative testing or trials may be needed where the substrate varies or product identity is uncertain. The findings should inform the method statement and quotation.
Cleaning and degreasing
Oil and grease can interfere with adhesion and may be spread rather than removed by abrasive tools. Suitable cleaning should therefore take place before mechanical preparation where contamination is present.
The cleaning product, rinsing or residue-removal method and waste arrangements should be compatible with the substrate and subsequent coating.
Mechanical abrasion
Power-tool methods can remove local corrosion, loose coating and create a key on suitable existing finishes. They are often useful on erected steel where full abrasive blasting would be disproportionate or difficult to contain.
Mechanical abrasion does not automatically achieve a blast-cleaning grade. The resulting condition must be assessed against the system’s actual preparation requirement.
Abrasive blasting
Blasting can remove mill scale, corrosion and existing coatings while producing a profile. Where Sa 2½ is specified, the visual cleanliness is assessed under ISO 8501-1.
Sa 2½ does not itself define a profile range, soluble-salt level or dust acceptance. Those requirements must be stated separately where relevant. See our Sa 2½ abrasive-blasting service.
Dry-ice and other cleaning methods
Dry-ice cleaning can remove suitable contaminants with little spent media, but it should not be assumed to remove firmly bonded corrosion or create a profile. It may form one stage of a combined preparation process.
Method selection should reflect containment, water sensitivity, occupied areas, waste, noise, access and the coating requirement.
Protecting the accepted surface
Prepared steel can quickly accumulate dust, moisture or flash rust. Inspection and primer application should be sequenced so the accepted condition is not lost before coating.
Hold points, lighting and access should allow meaningful inspection. Photographs can supplement records but do not replace stated acceptance criteria.
Preparation supports—but does not guarantee—performance
Good preparation reduces avoidable adhesion risks and helps the specified system be applied as intended. Fire performance still depends on the complete design, system selection, thickness, application, exposure and wider construction.
The ASFP technical-document library includes guidance on reactive coatings and their site application.
Frequently asked questions
Is cleaning enough before applying intumescent coating?
Sometimes for a sound compatible surface, but not universally. Corrosion, loose material, profile or overcoating requirements may call for additional preparation.
Which preparation method is best?
There is no single best method. Choose the least disruptive method that can demonstrably achieve the condition required by the selected system.
Can preparation be inspected after the coating is applied?
Most underlying conditions can no longer be assessed properly once covered, so inspection should occur at an agreed hold point before coating.
Discuss your project
For a preparation proposal, send photographs, existing coating information, access constraints and the proposed intumescent system.
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