METAL SUBSTRATES TROUBLESHOOTING PAINT PROBLEMS RELATED TO
METAL SUBSTRATES JUNE 2010
Description The formation of a powder on the surface of a weathered coating. The powders usually consist of broken down binder and freed pigment particles due to UV, temperature and moisture exposure.
Possible diagnosis • Different binders react at different rates, making it important to use the correct type and grade for the specific application.
• Interior quality paint has been incorrectly used for outdoor purposes.
• Low grade paint with low binder and high pigment loading has been used.
Remedy • Remove the friable chalk by scrubbing with stiff nylon or wire brush and rinse with water. High pressure equipment can also be used. Check for any remaining chalk after the surface had thoroughly dried. Repeat the cleaning process if powder is still present.
• If chalk persists, recoat with oil-based primer such as NEO Corrotec and recoat with appropriate quality top coat such as NEO Dur High Gloss (SABS Grade 2).
2. CORROSION BLISTERS
Description Fine blisters result in lifting and loss of adhesion from the substrate. Corrosion residues such as rust are often present inside the blister.
Possible diagnosis • Absence of a corrosion resistant primer under a topcoat.
• Insufficient coating system thickness.
• Failure to remove contaminants i.e. rust, salts, chlorides, oil and dirt prior to painting.
• Applying a coating system not suited to corrosive conditions. Corrosion-resistant coatings are generally less permeable, contain functional pigments and possess excellent adhesion and flow properties.
Remedy • Remove defective paint.
• Remove rust and contaminants from metal substrate.
• Select an appropriate coating for the substrate type and exposure conditions.
• Ensure that surface finish/texture is as per coating system requirements.
• Recoat surface in the prescribed time and during the correct weather conditions.
3. CORROSION UNDERCUTTING AT WELDS
Description Corrosion at welds.
Possible diagnosis • Painting over poorly prepared welded areas. The intense heat of welding leaves behind a thin oxidized carbon film layer at and around the welded area. This oxidized contaminant impairs paint adhesion.
• Weld imperfections such as rough welds, weld spatter or pin holes.
• Failure to remove weld scale.
Remedy • Remove all weld contaminants by sanding or grit blasting. Wire brushing does not always work as it only tends to polish the surface.
• Do not hesitate too long prior to painting to avoid the occurrence of flash rust.
4. FLASH RUST / MEASLE CORROSION
Description Measle like rust spots
Possible diagnosis • Rust removal and surface preparation of the steel surface before painting involved a lot of water.
• The de-rusted and cleaned bare steel surface was exposed to humid or damp atmosphere for more than 3 hours.
• A water-based or water-containing primer or top coat was applied to the cleaned metal and the rust appeared within 30 minutes of application.
• Very rough steel surface resulting in high peaks not adequately covered by coating.
Remedy • Remove paint and rust. Use flash rust inhibiting additive with water. Recoat immediately after surface preparation with NEO Zinc Chromate, NEO Corrotec or NEO Corroguard. Surface must be dry.
• When using water-based primers ensure that the paint contains appropriate rust inhibitor additives and/ or rust inhibiting pigmentation.
5. HEAT BLISTERS
Description Bubble-type blisters resulting in lifting and loss of adhesion from the substrate.
Possible diagnosis • Painting a warm surface i.e. radiators, steam pipes, steel roofs, furnaces/ovens etc.
Remedy • Remove the affected coating.
• Do not paint hot substrates.
• Use heat resistant coatings for surfaces subjected to extreme heat such as NEO Heat Resistant Aluminium.
6. PAINT PEELING / FLAKING FROM GALVANISED SUBSTRATE (Part 1)
Description The complete paint system is flaking and lifting from the substrate revealing the underlying galvanized surface.
Possible diagnosis • Failure to remove oil, wax and dirt from galvanized surface. A thin layer of oil is generally applied by the steel manufacturer to protect the metal from corrosion during the initial stages of transport and storage or substances are applied during the rolling and profiling stage of the product to facilitate the forming processes. Water-based coatings are more susceptible to delaminating when subjected to this type of surface.
Remedy • Remove old coating system.
• Remove oil and dirt contamination with NEO Metal Conditioner. This cleaner should be completely removable by means of water washing i.e. the solvent must be water-emulsifiable.
• Recoat with an appropriate coating system.
7. PAINT PEELING / FLAKING FROM GALVANISED SUBSTRATE (Part 2)
Description The complete paint system is flaking and lifting from the substrate. A
white powdery layer is present under the primer and on top of the
exposed galvanized surface.
Possible diagnosis • The use of an incorrect alkyd-based primer. During the drying phase the alkyd produces small amounts of acid byproducts which react with the zinc/galvanised layer, forming a white powdery salt layer. It is the powdery layer which prevents adhesion to the substrate.
Remedy • Remove the old coating system from the galvanized surface. Clean with a suitable abrasive degreaser such as NEO Metal Conditioner, and rinse thoroughly as described in Part 1.
• Apply a coat of NEO Corroguard Primer which will mop up the acid created during the alkyd drying process.
• Overcoat the primer within 16-24 hours.
8. PAINT PEELING /FLAKING FROM GALVANISED SUBSTRATE (Part 3)
Description The complete paint system is flaking and lifting from the galvanised
substrate. Dark grey flecks adhere to the back of the primer.
Possible diagnosis • Poor quality galvanized coating or one or two pack acid primers have been applied. The primer etches into the topmost layer of the zinc. When subsequent coats (undercoat/topcoat) are applied they produce stress in the zinc film which results in delamination.
Remedy • Galvanised coating with poor integrity is a difficult substrate to overcoat.
• Follow the recommendations as outlined in Part 2 & 4.
• Avoid using an acid etch primer on galvanised or zinc coated substrates.
9. PAINT PEELING / FLAKING FROM CALCIUM PLUMBATE PRIMER (Part 4)
Description Loss of adhesion of a solvent or water- based topcoat from a calcium
a.The topcoat peels off after a few months after exposure to hail
or severe rain.
b. As “a” but the de-lamination of the topcoat occurs almost
Possible diagnosis (a) The calcium plumbate primer was left for more than 48 hours prior to over coating. The calcium plumbate primer has cured too hard and the topcoat has not been able to bond adequately to the primer.
(b) A mixture of (a) and possibly a contamination of dirt and oil on the primer prior to applying the topcoat. Water-based topcoats are more susceptible.
Remedy • Remove the loose and flaking coating. Clean, degrease with NEO Metal Conditioner, wash off with water, then lightly sand down and reapply topcoat.
10. PAINT PEELING / FLAKING FROM WATER BASED PRIMER
Description Loss of adhesion of an alkyd solvent-based topcoat from a water-based primer after approximately 7 days of drying.
Possible diagnosis • The hydrophobic alkyd solvent-based topcoat does not adhere to the hydrophilic water-based primer. When the alkyd paint starts to cure and harden, a tension is created in the film which pulls the alkyd topcoat away from the primer.
Remedy • Preferably water-based primers should be used with water-based topcoats and similarly for solvent-based systems.
• Remove loose and flaking paint and recoat with appropriate top coat.
Description The development of wrinkles in a film surface during drying.
Possible diagnosis • Too thickly applied oil-based paint, usually due to the formation of a surface skin, whilst the underlying layer takes longer to cure and is still soft.
• Incorrectly formulated oil-based paint with too much surface drier ingredient.
Remedy • If the coating is still slightly wet, remove with NEO Paint Remover and recoat in a thin, even layer under the correct temperature.
• If the wrinkled coating has fully dried, sand down and recoat in the correct manner.