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I. CORRECTING COATING PROBLEMS, Loss of Adherence (Spalling and Chalking)
A. DURING DRYING AT ROOM TEMPERATURE
1. Unclean Surface
Clean the surface of oils, scale, dirt, etc. This will greatly improve coating adherence. Not cleaning the surface is analogous to coating a freshly-sawed board ... over its sawdust ... whereby the paint would adhere to the sawdust and not to the board.
2. Surface Incompatibilities
If the paint is applied over another coating or onto unfired ceramics or graphite, there may be an interaction between the binders used -- causing the coating to spall or a normally hard and scratch-resistant coating to chalk and become soft and dusty. This is often solved by first heating the component to be coated to about 330C (570F) or higher to drive-off the binders -- then allowing the component to cool and then coating.
3. Excessive Dilution of the Paint
Many formulations are designed to yield a soft coating. Dilution of these coatings is not generally detrimental.
Some formulations are designed to produce a harder, more wear-resistant layer. Dilution of these coatings will often cause the coating to become soft. See AIR-SPRAY section for spraying viscous paints without dilution.
B. DURING HEATING TO SERVICE TEMPERATURE
1. Heating of wet coating
If the coating spalls during the heat-up cycle, this is almost always due to the coating being wet (even if it looks and feels dry) when exposed to heat.
2. Surface Incompatibilites
Please see Section A2 above.
C. DURING COOLING FROM SERVICE TEMPERATURE
1. Expansion/Contraction Differences
If the coating spalls during cool-down, the cause is generally due to a mismatch in expansion/contraction coefficients between the coating and substrate. This is particularly true at temperatures above 600C (1100F) and for coated metals, and with the harder, scratch resistant coating formulations. Soft coatings and thin layers are less likely to spall.
2. Thick Coatings
The paints should be applied like housepaint - as a thin layer. Thick layers of paint are much more likely to spall on cool-down.
II. APPLICATION NOTES - SURFACE PREPARATION
A. METALS
Coating performance is directly related to proper surface preparation. Coating integrity, adherence and service life will be reduced if surfaces are improperly prepared. Nearly 80% of all coating failures are attributed to inadequate surface preparation.
Coatings are formulated to perform well using a one-stage cleaning operation. Cleaning is designed to remove surface oils and contaminants, dirt, rust, scale, dust, etc.
1. Blasting
Blasting is probably the best cleaning method. Not only does blasting remove the surface contaminants, oils, scales, etc., it roughens the surface -- increasing coating adherence.
2. Burn-Off
"Burn-Off" is designed to burn-off oils by heating to 400C (750F) or higher. This still leaves a residue that must be removed, generally by mechanical means (blasting or mechanical cleaning).
3. Mechanical Cleaning
A "Mechanical Cleaning" is often used to remove residues from burn-off or scale, dirt, etc. prior to solvent cleaning.
4. Solvent Cleaning
A strong, metal-cleaning solvent is generally required for effective cleaning.
B. CERAMICS AND GRAPHITES
Ceramics and Graphites are generally more forgiving to coat than metals in terms of surface preparation. To prepare surfaces, remove loose dust and any obvious contaminants.
Unlike metals, ceramics are often porous. For very porous ceramics, the water or solvent in the coating is absorbed so fast into the ceramic that the paint cannot be smoothed during brush application. To avoid this situation, it may be necessary to dampen the surface with water prior to brushing or to apply the paint by air-spray techniques.
When coating "green" or unfired ceramics and graphite, there is sometimes an adverse reaction between the binders in the coating and the binders in the ceramic or graphite. This reaction can cause the coating to chalk (a coating that is normally hard becomes chalky) or to spall and peel from the surface. This is seen upon drying or after firing.
To correct this, pre-fire the substrate to be coated to burn-out the binders.
III. APPLYING THE PAINT
A. BRUSHING
1. Re-suspend the paint before use.
Most formulations re-suspend easily by hand shaking. Some may require rolling overnight or extended mixing, especially after long-term storage
2. Apply a thin, uniform layer of paint -- just like ordinary housepaint.
Generally, a foam rubber brush is best, especially for more viscous paints.
Avoid thick layers of paint. They tend to crack on drying and are much more likely to spall during cooling from elevated temperatures.
3. Allow to thoroughly dry at room temperature before heating. If forced-dried, do so below 70C (160F).
Heating of "wet" coating will cause the coating to spall.
Note: brushing onto a substrate heated to around 40-50 C [100 to 125 F] can facilitate drying; however if the substrate is too hot, a brush can 'drag'/'glue' down to the substrate.
B. AIR-SPRAY
Please see attached hints for effective air-spray techniques.
C. DIPPING
Dipping is generally not recommended.
Dipping normally produces a very thick, non-uniform layer of paint that often cracks on drying and spalls after heating.
IV. AIR SPRAYING TIPS
A. SURFACE PREPARATION
1. If possible, use a heated substrate between 150 and 200C [300-400 F] for spraying water-based coatings. Air-spraying a water-based coating onto a heated surface will lead to immediate drying, such that the coating goes down like if using an aerosol can.
2. A roughened substrate improves adherence.
3. Clean the substrate very well!!
B. AIR SPRAYING
Use the attached recommended spray equipment. If other equipment is preferred, be sure to use a pressure-feed system, since a siphon-feed system will not allow most paints to be sprayed properly due to their viscous nature. Also, if other equipment is used, it is good to request an 'equivalent spray nozzle' to DeVilbiss' 704FF. Avoid heavy spray coats. A 2 to 4 mil layer is ideal. Build up the thickness slowly to avoid running. Use short bursts rather than a continuous spray. This allows maximum control during spraying. Maintain a distance of at least 6 to 12 inches from the substrate to permit uniform coating. Too close of a position with the spray gun causes too heavy of a coating. Maintain proper gun and tank air pressure. Too high or too low pressure causes spotting of the coating. When these are not correct, the paint sprays as small blobs rather than a mist. Use a sweeping motion for the gun and change the spray angle often in order to cover all pores, crevices, etc. Do not hold in one spot while spraying. Start the spray while not directed at the substrate when possible. Air guns often shoot out a heavy stream in the beginning until the correct air/coating mix is achieved. Adjust the spray cone angle for the best coverage using the gun adjustments. Always clean the equipment after use.
C. SAFETY NOTES
Use appropriate safety equipment. Refer to the Material Safety Sheets for details.
V. RECOMMENDED EQUIPMENT
If other equipment is used, it is good to request an 'equivalent spray nozzle' to DeVilbiss' 704FF.
COMPANY:
ITW- Industrial Finishing Headquarters
195 Internationale Blvd.
Glendale Heights, IL 60139
Telephone: 888-992-4657
FAX: 630-237-5011
Website: http://www.devilbiss.com
CATALOG NO. DESCRIPTION
JGA-510-704-FF Hand Spray Gun, Air Tip, Needle
TSC-591 1 qt. Pressure Feed Cup
KK-4980 Regulator for Cup Air
FOR LARGER USAGES, substitute DEVILBISS:
JGA-510-704-FF Hand Spray Gun, Air Tip, Needle
KB-555 2 qt. Remote Pressure Feed Cup
KB-4006 Connector kit, includes all hoses and fittings
SUGGESTED DISTRIBUTOR:
C&C Industrial Sales
2154 Utopia Ave.
Nashville, TN 37211
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