Shot blasting removes mill scale for steel.
Mill scale is a type of iron oxide that is formed on the surface of the steel during the hot rolling process and is porous. The very high surface temperature combined with high roller pressures result in a smooth, bluish grey surface. If mill scale were a uniformly well-adhered covering to the steel section, then it would make an ideal protective barrier. Unfortunately, mill scale is not uniform, nor is it well-adhered.
This latter item is of concern in that many projects have had high quality, expensive protective coatings applied directly over mill scale and inadequately prepared steel only to have the entire structure and coating system fail within a very short period of time.
The mill scale is fact looks like a scale and it can “pop off” the surface, cracking the coatings of paint and allowing moisture to penetrate. This allows a galvanic reaction to occur which results in pitting corrosion (rust) to the base steel.
This will result in a growth of the corrosion under the scale and produce further cracking to the coatings and expose greater areas and thus more corrosion.
All mill scale must be removed to present a uniform and clean surface.
At Bluewater Boats our steel surfaces are cleaned to a SA 2.5 profile with an abrasive blast. This will remove the mill scale and provide a surface profile that will give the paint system its design requirements.
Painting over mill scale, however tempting, is a futile exercise, as the presence of mill scale on the steel surface accelerates the corrosion of the underlying steel.
Below are some photos taken of a new sailaway built by another boat builder. The owner brought it to us for shot blasting and re-painting as the rust was forming, both under the water line and above at an alarming rate! This boat is not even 12 months old when it has had to be taken out of the water and delivered to us for some much needed restoration work. The photos underneath are of the newly painted shot blasted narrowboat.
TWO PACK PAINT SYSTEMS
TWO PACK BLACKING
Upon completion of shot blasting to Sa 2.5 we apply two coats (wet on wet) of a two pack epoxy blacking Interzone 954 up to the top rubbing strake (except base plate).
Interzone 954 is the ultimate high performance two pack epoxy blacking system for the leading paint specialists International Paint. Interzone 954 was formulated as a splash zone product and is used on high profile offshore installations globally.
Upon completion of shot blasting to Sa 2.5 on the cabin sides, roof and deck areas we apply a two pack high build, high performance epoxy primer, a two pack epoxy finishing primer, polyurethane undercoats, non slip coatings and polyurethane top coats in a wide range of colours.
The benefits of shot blasting and a two pack paint system are:
Interzone 954 is a harder wearing blacking
Interzone 954 can be guaranteed for seven years
If Interzone 954 is scratched below the waterline, due to the indentation of the blast profile the damaged area will not spread unlike most conventional paint systems
TWO PACK PAINT SYSTEMS
Two pack polyurethane
Two pack polyurethane provides longer lasting wear
Two pack polyurethanes are abrasive resistant
An ultra high gloss with exceptional colour and gloss retention
Longer lasting results – contains a unique package of HALS/UV† absorbers together with a chemically cured polyurethane resin system which protect against premature ageing
Exceptional durability – gives prolonged impact & abrasion protection even on the highest wear areas, i.e bollards and fender areas
Two pack acrylic urethane
Awlcraft 2000 is a two component, fast drying acrylic urethane coating with long lasting gloss and colour retention.
Provides an easy to apply, buffable finish, i.e. for small areas of tree damage, scratches can be buffed and polished back to its original condition
Awlcraft 2000 is also available in a range of metallic colours
Both of the above two pack systems are more commonly applied to Sunseeker and Princess super yachts giving you an excellent paint finish, making sure your boat is the envy of your friends.