The primary reason for surface preparation prior to application of a protective coating is to create a surface that will optimize adhesion of the coating. This is accomplished by maximizing each of the three requirements – Clean, Dry and Rough.
Let’s examine each of these items for each type of substrate:
Definition: Free of contaminants, both visible and invisible.
- Rust, Scale - remove by grinding, wet or dry blasting or water blasting
- Old Coating – remove by grinding, wet or dry blasting. NOTE: if prior coating is a 2-part system and is well adhered [withstands blasting] it may not be necessary to remove the coating, only texturize it – see “Rough” below]
- Oils, Greases, Waxes, etc. – remove by degreasers. Apply final treatment with alcohol, acetone, MEK to remove any residue
- Soluble salts [Chlorides, Sufates]
– remove by hot water or steam. Flash rust is a concern.
On Concrete and Tile:
- Oils, Grease – remove by grinding, wet or dry blast or shot blast, followed by degrease as needed.
- Old Coating – see Metal/Visible above
- Clear Sealer – test by putting small amount of water on the concrete. If the water stays on the surface, a clear sealer has been applied. Remove by grinding, wet or dry blasting, shot blast.
Definition: A surface that, versus a smooth surface, creates additional area that allows for:
- Additional bonding sites and
- Increases the mechanical adhesion of the coating.
- Abrasive blasting [dry or wet] to an SSPC 10 near white metal blast is the first recommended surface preparation method. A minimum 3 mil profile is recommended. A hard blasting aggregate, Aluminum oxide, Slag [Black beauty], Garnet or other suitable aggregate is required. Consult your blasting contractor as to the system to achieve the required 3-mil profile.
- When blasting is not an option, aggressive diamond grinding may be used.
- Moderate grinding/sanding to break any existing glaze is recommended.
- Concrete finished by floating, especially magnesium floats, can have the surface pores closed to an extent that a coating will not well adhere without prior treatment. Grinding, high pressure water blast or shot blasting is recommended to open the pores and roughen the surface.