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Mild Steel Surface Preparation

The standard DuluxProtective Coatings recommendation for the preparation of new steel prior to coating application isabrasive blast cleaning according to AS1627.4 Class 2 Near White Metal blast cleanliness, to replicate visualstandard Sa 2 in AS1627.9, and to generate an angular surface profile of 30 to 60 microns. Details of this process,and the reasons for doing so are described in Dulux Protective Coatings Tech Note 1.1.2 Mild Steel SurfacePreparation. Suffice to say that this method consistently results in a clean and well-profiled steel surface, providing a near-perfect base for just about any heavy-duty two-pack system to adhere to.

If, however, the steel is to be cleaned to remove all traces of existing coatings and surface rust, and recoated, and if

abrasive blast cleaning is not convenient, then UHPWJin conjunction with Dulux Luxepoxy Sealer is certainly atried and tested alternative.

UHPWJ What is it?

UHPWJstands for Ultra High Pressure Water Jet, which isthe process of cleaning a surface using water under very highpressure (around 35,000 psi).

The pressure of the UHPWJcan be adjusted to provide sufficientforce to remove all surface contaminants, including tightly adheringsurface rust, leaving the surface completely clean.

To ensure that the surface is in fact sufficiently clean to receive acoating system, a visual assessment may be made by comparingthe cleaned surface with a series of photographic standardsissued by either NACE (National Association of CorrosionEngineers) or SSPC(Steel Structures Painting Council, now Societyfor Protective Coatings).

Can UHPWJ Replace Abrasive Blast Cleaning For New Work?

The answer is no, because abrasive blast cleaning is just about the only method that consistently removes millscale,and it provides a good surface profile, whereas UHPWJ does not remove millscale nor create a surface

profile.

The performance of a coating system largely depends on bond strengthbetween the clean steel substrate and the first coat (or primer). Coatingfailuresometimes occurs at the steel primer interface, hence theimportance of giving the surface a profile to maximise the bond area.The greater the profile, the greater the surface area and the strongerthe mechanical and chemical bond.

If the surface of the steel is smooth, however, then adhesion of thecoating is entirely dependent on a chemical bond, which can varyenormouslyfrom product to product. Whilst abrasive blast cleaning

provides a good surface profile, UHPWJ does not create aprofile at all.

If any millscale is present, then delamination is also very likely to occur. For more information about millscale, pleaserefer to DuluxProtective Coatings Tech Note 1.1.4 Millscale.

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Can UHPWJ Replace Abrasive Blast Cleaning For Maintenance?

The answer is yes,mostly, for maintenance and repaint work over steel that had originally been abrasive blastcleanedand you intend to prime the steel with Dulux Luxepoxy Sealer . Heres why.

Steel that had been originally blast cleaned and painted, can be UHPWJ cleaned to remove all the existingcoating and surface contaminants because it effectively removes contaminants and reveals the original profile, whichwill be just as suitable for painting as it was when originally blast cleaned. Therefore, the all-important profile is alreadythere ready for the new coating system to key into.

Use of Dulux

Luxepoxy

SealerIndependent testsi have shown that the bond strength of Luxepoxy Sealer to UHPWJ cleaned steelconsistently exceedsthe minimum recommended for epoxies to newly blast cleaned steel.

The adhesion test results of Luxepoxy Sealer , applied at 20 microns, alone and as part of two different coatingsystems are summarised in the table below.

Description

Bond Strength

MPa)

Bond Strength

psi)

LuxepoxySealer@20m 15.5 2,330

LuxepoxySealer@20mplusDurebildSTEGlassFlakeat200m 13.5 1,980

LuxepoxySealer@20mplusDurebildSTEGlassFlakeat200m

plusWeathermaxHBRat65m12.7 1,850

Adhesion values in excess of 3 MPaare generally considered to be goodforepoxies applied to newly blast-cleaned steel.

The adhesion values for the Luxepoxy Sealer coating systems toUHPWJ

cleaned steel in this test averaged 15.5 MPa (2,250 psi), demonstrating thatUHPWJin combination with Luxepoxy Sealer is a viable alternativeto abrasive blast cleaning in maintenance situations.

This is handy information for maintenance projects where UHPWJwould bemore practical than abrasive blast cleaning, and the steel requires theapplication of a high performance coating system.

For more information, please contact the Dulux Protective Coatings TechnicalConsultant in your state.

iCTIConsultantsReportNo.C10547,CTIJob2085,12January2008.