Painting a bridge or an industrial structure is an expensive operation. Now more than ever, state DOTs and facility owners are specifying warranties and application methods to ensure the longest possible life for their coatings. Determining what factors affect paint cure can help to achieve the optimum performance and provide the owner and contractor with fewer headaches down the road. As always, for any coating to be successful, a good specification, quality contractor, proper equipment and of course an effective inspection program are all necessary components. Once paint is applied we need to wait a specified period of time to let the paint pass from a liquid state to a solid state. This is commonly known as drying time. Contractors, coating inspectors and owners speak of drying, hardening and curing of a coating, but it is important to clarify each of these coating states as they mean different things and occur at different times in the process.
DRYING TO THE TOUCH FOR SOLVENT AND WATER-BASED COATINGS
The paint-drying process corresponds to the evaporation of all solvents and diluents added to the paint in order to make it a liquid. Paint “dry” happens when the solvents evaporate from the coating, leaving the paint feeling dry to the touch even though it is not 100-percent cured. Drying time is sensitive to temperature and humidity. For most coatings, to be “dry to the touch” is not a sufficient state over which to apply another coat. Product data sheets must be consulted to ensure proper application.
HARDENING (DRY TO RECOAT)
The hardening of a coating corresponds to the process by which the main polymer of the coating hardens with all the other pigments and additives that make up the paint, creating a solid and adherent coating. Hardening does not necessarily mean curing. Hardening is, however, most often the state at which recoat is acceptable.
CURE (COMPLETE CURE)
The paint-curing process corresponds to both the drying and the hardening processes. For example, a coating can be dry to the touch 30 minutes after application but not hardened enough to recoat until two hours after application. And still, the coating may not be finished curing until seven days after application. Considering these definitions, it is important to understand that when using a two-component coating such as an epoxy polyamide or a polyurethane, and a hardener is not added, one can observe over time that the paint feels dry to the touch, meaning no smudges result, but has not hardened, still allowing easy removal of the applied coating. We can say then that the paint has dried (the solvent has evaporated), but it has not hardened, due to hardener not being added, and hence the paint has not cured. Other issues that might cause drying or curing problems of a coating are:
- Problems with the coating material sent from the manufacturer.
- Use of the wrong thinner or contaminated thinner.
- Moisture in some generic thinners compromising paint cure.
- Coating film thickness.
- Problems related to the type of surface to which the coating is applied, for example, wood absorbs paint whereas paint lays on top of steel.
- Temperature considerations when mixing or storing coatings.
- Ambient conditions present during dry and cure times.
HOW TO CHECK IF YOUR PAINT IS DRY
To check if your paint is dry, touch an inconspicuous area with your finger. If the paint is not tacky and feels dry to the touch, it is dry, but as stated earlier, it is not necessarily cured.
HOW TO CHECK IF YOUR PAINT IS CURED
To check if your paint is cured, use the fingernail test. In an inconspicuous area, press your fingernail into the coating. If it leaves an indent, your paint is not fully cured. If no indent is visible and the surface is hard, your paint has most likely cured. The most critical aspect of applying any coating is the review, understanding and familiarity of the information presented on the paint manufacturer’s product data sheet prior to application. The product data sheet provides all the necessary information regarding surface preparation, application conditions, application equipment, mixing instructions, application procedures, spreading rate, drying schedule, clean up, performance tips and safety precautions. The principal conditions that have the greatest effect on the cure of a coating are temperature and humidity.
Air temperature and steel (surface) temperature affect solvent evaporation, brushing and spraying properties, drying and curing times and the pot life of two-pack and three-pack materials. If heating is required, this should be accomplished by indirect methods only.
Paint should not be applied when there is condensation present on the steel surface or the relative humidity of the atmosphere is such that it will affect the application or drying of the coating. Regular practice involves measuring the steel temperature with a contact thermometer to ensure that it is maintained at at least 5 degrees (F) above the dew point. Following are some conditions that can occur if a coating is not properly dried or cured.
BLUSHING (AMINE SWEATING)
If cured during cool ambient temperature conditions, dropping temperatures or high humidity, amine-cured epoxy resin coatings can develop a surface oiliness or exudates, commonly referred to as “amine blush” or “sweating.” This is caused by the absorption of carbon dioxide and water into the coating film and its reaction with the amine curing agent. Some of the resulting problems can be surface tackiness or greasiness, incomplete cure, poor adhesion, poor adhesion on overcoating, coating discoloration over time and poor gloss retention.
Cratering is the formation of small bowl-shaped depressions in the coating caused by air trapped in the film. The air forms a bubble that bursts, leaving a crater. Cratering is common in coatings that are roller- or brush-applied, often by an inexperienced applicator. Improper mixing procedures can also trap air in the coating causing cratering.
Orange peel is the term for uneven film formation (with hills and valleys) that results in an appearance resembling the skin of an orange. This defect can be caused by improper application techniques where the spray gun is too close to the surface or the air pressure is too low to ensure proper atomization. Hot air temperatures may also cause this defect because paint droplets dry before they can flow out and fully level evenly together.
This defect is the presence of a group of small furrows or ridges in the surface layer of paint, found mostly in alkyds and other oil-curing coatings. It is caused primarily by the action of solvents or reducers in the previous layers, which either react with the substrate or are trapped and then try to escape to the surface, lifting and swelling the last layer of paint. A surface skin forms that prevents the underlying binder from curing any further. When the skin surface contracts, the film wrinkles.
Blistering can occur when there is a remainder of air, solvent or other contamination between the topcoat and the previous layers of paint causing the paint to lift and swell, thereby forming blisters. The primary difference between blisters and other types of defects is the characteristic circular shape and the absence of a puncture in the finish paint.
Most coatings are made for application without the need to add thinner. However, when trying to apply coatings in lower temperatures it may be necessary to add thinner to reduce viscosity for an effective application. Adding thinner may cause the coating to take longer to cure or cause it to curdle. Always add the type and amount of thinner recommended by the paint manufacturer and remember to blend multi-component paints together thoroughly first before adding thinner.