Surface Prep.pdf

General Rules for Surface Preparation:

Surface Prep is the most important factor when considering a coatings project. When systems fail, the first thing the inspector will examine is the quality of the surface prep.

1. Abrasive Blast Cleaning

The Coating Guidelines are based on abrasive blast cleaning of surfaces, using standards published by the Society for Protective Coatings, formerly the Steel Structures Painting Council (SSPC)1.

Roughly equivalent standards are also published by the National Ass’n of Corrosion Engineers (NACE) and the Swedish Institute of Standards (SA).

MARAD uses SSPC definitions in its specifications.

The various grades of blast cleaning are:

SSPC SP-6: Commercial Blast Cleaning SSPC SP-7: Brush-Off Blast Cleaning (Sweep) SSPC SP-10: Near White Metal Blast Cleaning SSPC SP-5: White Metal Blast Cleaning (new constr only)

These can be considered performance specifications, in that the SSPC standards define the quality of the surface once preparation is complete, but they do not specify the means to achieve the surface. There are many types of blast media, some which can be recycled, and others which cannot. In general, MARAD specs do not specify the type of abrasive to be used, or the method of blast cleaning. That is left to the shipyard or contractor to decide.

2. Hydroblasting (Water Jetting)

Hydroblasting as a means of surface preparation is rapidly gaining in popularity, and effectiveness. You will likely see specifications invoking the use of hydroblasting becoming more common in the RRF as the technology matures.

Typically, MARAD hydroblasting specs tend to specify as nozzle blast pressure (ex 35,000 psi) at a distance (typ 12") from the surface to be cleaned. Before the development of industry standards this was the only way to specify hydroblasting. This has now changed. A joint SSPC/NACE standard was published in 1995, and should be the standard MARAD reference.

This standard is a comprehensive equivalent to the abrasive blast standards. It defines industry terminolgy for the various types of Water Jetting (this is the preferred terminology), and surface

1 The Society for Protective Coatings has retained the "SSPC" acronym because of its widespread familiarity.

cleanliness standards which are roughly equivalent to the four abrasive blast standards. In summary these are:

SSPC SP-12 / NACE No. 5 Surface Preparation and Cleanliness of Steel and Other Hard Materials by High and Ultrahigh-Pressure Water Jetting Prior to Recoating

Terminolgy:

Low-Pressure Water Cleaning: less than 5,000 psi High-Pressure Water Cleaning: 5,000 - 10,000 psi High-Pressure Water Jetting: 10,000 - 25,000 psi Ultra-High Pressure Water Jetting: above 25,000 psi

Surface Cleanliness Rough Equivalents

WJ-1 SP-5 White WJ-2 SP-10 Near White WJ-3 SP-6 Commercial WJ-4 SP-7 Sweep

A proper specification should reference SSPC SP-12, and specify one of the WJ surface cleanliness conditions. Generally the contractor should decide whether to use High or Ultra-high pressures, in the same manner that the contractor chooses which type and method of abrasive blasting to use.

Another acceptable standard is published by the International Paint Company. Their standards are roughly equivalent to SP-6 and SP-10, and also specify degrees of flash rusting after hydroblasting. Light flash rust is generally acceptable with surface tolerant epoxies, even in immersion service. Moderate and heavy flash rusting must be removed prior to painting.

International Hydroblasting Standards

HB2 SP-6 “Thorough Hydroblast Cleaning” HB2-1/2 SP-10 “Very Thorough Hydroblast Cleaning”

L Light Flash Rusting M Moderate Flash Rusting H Heavy Flash Rusting

Hydroblasting/Water Jetting is an acceptable means of surface preparation for exterior hull surfaces, including the underwater hull. It can also be used in cargo holds. It is discouraged for tanks because of a relatively poor commercial track record. If specified for tanks, special precautions (primarily ventilation and dehumidification) need to be specified.

General Rules for Hull Coating

1. Generic Coating Systems Requirements

This is the basic guideline for the types of surface prep and coating materials to be used in the various areas of the ship.

It has not been updated to reflect the use of hydroblasting as an acceptable means of surface preparation. Generally, for exterior and interior (excl tanks) applications, substitute the list abrasive standard with its Water Jetting equivalent. Most of the epoxy primers listed are suitable for use over a hydroblasted surface. If a particular product is not suitable, a substitution can be made.

2. When to Replace, and when to Repair

As a general rule of thumb, epoxy coating systems in have an expected lifespan of between 10 and 15 years. So at 15 years, regardless of condition, it is a good idea to completely replace a system.

On underwater hulls in the 10 year program, complete replacement should be performed at each drydocking (assumed to be at 10 years), and the ship should be fleeted. Ships on a 5 year inspection interval can go 15 years before complete replacement, and do not need to be fleeted. They do, however, need to rotate block positions at each drydocking (usually docking plans show 2 or 3 block positions for this purpose).

When maintaining an underwater hull coating system, you generally will spot repair areas of failure and/or mechanical damage, and then overcoat with additional anti-fouling. S/M’s and some Surveyors will want to sweep blast (SP-7) to save money. This is penny-wise, pound-foolish. Sweep blasting damages the surface of epoxy coatings, and generally leads to premature failure of an otherwise intact coating. It should be discouraged. It may be appropriate to sweep off anti-fouling coatings (which can otherwise be difficult to remove), and then prepare the intact epoxy by Water Jetting.

Tank Coatings are generally replaced when failure reaches 30 percent of the surface. Less than 30 percent must be evaluated on a case-by-case basis, but generally this condition can be maintained by using a maintenance coating (NOT A SOFT COATING), or if in a fresh water tank (NOT POTABLE), by the use of a corrosion inhibitor (usually sodium silicate). Tanks can also be held by installing sacrificial anodes, but these will only be effective when the tanks are ballasted with salt water.