Many factors contribute to the end results of industrial finishes. One of the most important of these is surface preparation. With poor surface preparation, the life of any industrial coating is severely compromised; and, conversely, proper surface preparation will enhance the appearance and durability of any coatings operation.

With the advent of today's technology in high solids and waterborne resins, along with the wider variety of substrates being coated, it becomes increasingly important to pay close attention to surface preparation. While these newer resins offer many superior physical properties, they are also less forgiving than the conventional resins when applied over a marginally-clean substrate.

There are many options available in today's market. Metal cleaning and chemical treatment methods range from solvent wiping a part to the five, six, or even seven-stage chemical pre-treatment operations. It is essential that you determine what method or combination of methods is most feasible and effective for your operation.

Options For Metal Cleaning:

• Solvent Cleaning:
  • Wiping
  • Solvent-Jet Cleaning
  • Vapor Degreasing
• Alkaline Cleaning
• Emulsion Cleaners
• Acid Cleaners
• Burn-Off
• Mechanical Cleaning (scraping, wire brushing or sanding)
• Blasting (with sand, shot or grit)

In many finishing cycles, there is a lapse of time between cleaning and subsequent painting. It is an unfortunate circumstance that clean metal often rusts more rapidly than dirty metal. As a result, some means of controlling flash rusting is necessary. Where cleaning is followed by a metal treatment in a continuous system, no problem exists. But if no subsequent treatment is used, it is necessary to protect the metal if there is a delay before painting the product. There are numerous proprietary compounds available for this purpose. Contact a chemical/pre-treatment supplier for the best recommendations for your specific operation.

It is now standard practice to chemically treat metals after cleaning to further improve adhesion, corrosion and humidity resistance. It is critical that you contact a reputable chemical/pre-treatment supplier to assess the best method of treatment and the best combination of chemical products needed for your type of substrate. Certain metals such as zinc, aluminum and magnesium require special handling.

The proper choice of cleaning and treatment systems must be made by the manufacturer of the items to be coated. Not only do the different methods vary in efficiency, but the number of stages through which the work is put will affect the final results.

Some of the many factors that must be considered are:

1. The kind of metal (steel, aluminum, zinc, etc.)
2. The kind of contaminant (rust, dirt, grease, etc.)
3. The ultimate durability required of the finish.
   • Will the item be exposed to high humidity, salt atmospheres, etc?
   • Is it a short or long-life item?
   • Is the finish purely decorative, or must it also protect?
4. Initial cost.
5. Space available.
6. What are your competitors doing?

These points are of such scope and involve basic policy to such an extent that only those responsible for the manufacture and sale of the article in question can give them proper weight consideration.

Some Cleaning-Treatment System Options:

One-Stage - cleaner

Two-Stage Operation - normally a cleaner in the first stage, followed by a rinse.

Three-Stage Operation - normally a cleaner in the first stage, followed by a rinse in the second stage, followed by a rinse with a chemical added to aid in rust prevention.

Four-Stage Operation - first stage cleaner, second stage rinse, third stage a phosphating solution, fourth stage rinse.

Five or Six-Stage Operations - first stage clean, second or second and third stages rinse (one or two rinse stages, depending on efficiency of the rinse), third or fourth stage is phosphating, fourth or fifth stage is rinse, fifth or sixth stage would be a chemical rinse. Five and six-stage systems are normally for appliance coatings, etc., where maximum paint performance is required.

Summary:

1. The object of cleaning the metal is:
   1. To condition it for subsequent metal treatment.
   2. To remove all contamination, dirt, oily residue, etc., which prevents proper adhesion of paint.
   3. To remove all soluble salts, sulphates and chlorides in particular, which promote blistering and rust on exposure. Hardness of the rinse water must be considered since the salts present may themselves cause premature blistering.
2. When using chemical cleaners, thorough rinsing is mandatory. A cold overflowing rinse is generally preferable to a hot still rinse. The final rinse in any system should never be alkaline. Preferably, it should be acidified.
3. Metal treatment is used to:
   1. Improve adhesion.
   2. Improve durability by increasing corrosion resistance.
4. Chemical cleaning can be done either in power-spray washers or dip tanks. Different chemicals are usually used and the manufacturer's recommendation should be followed.
5. Rustproofing compounds that leave alkaline, soluble salt or oily deposits are not satisfactory to paint over.
6. To provide a barrier between the metal and subsequent paint coats which will prevent a possible reaction between the metal (e.g. zinc) and the paint vehicle which results in loss of adhesion.

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