Author: Phil Stevenson
Have you ever had a sample of crackle finish handed to you by a customer? The finisher’s first reaction is most often one of fear and apprehension at the thought of having to produce a crackle finish. These reactions may be well founded. Crackle finishes are truly one of the most difficult finishes to achieve consistency and predictable results that are aesthetically acceptable. Upscale consumer selections are trending towards these types of complex crackle finishes. To be prepared for these occasional crackle requests, the finisher must understand the science behind these coatings to minimize the variables that affect the final result.
Types of Crackle Systems
There are several types of crackle systems to choose from: the latex base system, glue and paint base systems, and the lacquer base system. For use by the industrial finisher, the lacquer base crackle finishing systems are most suitable for a production environment and will be the only system discussed throughout this article.
How Do Crackle Finishes Work?
Lacquer crackle systems require a nitro-cellulose lacquer to be applied as a first base coat. A second coat of crackle lacquer is then applied over the base in a contrasting color. The base coat and crackle lacquer are formulated with different unequal surface tension that allows the crackle lacquer to “pull” and shrink over top of the nitro-cellulose base coat, creating the crazed crackle effect. Crackle lacquers are formulated with high levels of inert ingredients to create this unequal surface tension and are very porous by default, thereby exhibiting poor adhesion and low levels of moisture, chemical, and scratch resistance. Because of these inherit characteristics, the crackle lacquer must be “locked down” with pre-catalyzed lacquers, post-catalyzed lacquers, or conversion varnishes to provide sufficient protection and overall performance of the system. Unlike other standard finishing schedules, it is normally acceptable to apply a conversion varnish clear coat over the crackle lacquer without incurring a compatibility issue. However, it is recommended that you consult with your finish supplier prior to finalizing your finishing schedule.
Controlling Crackle “Crazing” Effects
The most difficult aspect of crackle finishing is controlling the size and shape of each “island” of crackle. Finishes that have cracked due to natural aging processes will crack larger in some areas than in other areas of the wood. This natural cracking phenomenon is pleasing in appearance and should be replicated by the finisher. With the exception of a very rustic look, the finish should not exhibit large “islands” of crackle along with small “islands” of crackle within the same piece. This inconsistency will often be too jarring and bold to be visually pleasing, especially in larger quantity or on smaller objects. To gain control over the crackle operation, the finisher must consider the following variables:
- Moisture Content of the Wood.
Moisture in the wood affects the amount of absorption of the finish into the pores of the wood. Low moisture content results in less cracking, while high moisture content results in less penetration into the wood allowing more crackle to occur.
- Consistent Sanding.
Uniform sanding with the grain is imperative! Crackle lacquer will follow the scratch pattern in the wood created by the sanding process.
- Wood Specie
If using paint grain wood, be sure the same specie of wood is used throughout the project unless varying levels of “crazing” is desirable. Each specie of wood will have its own characteristics of crackle effect due to the differing grain textures and density of the wood.
- Consistent Coating Thickness
Applying consistent mil thicknesses of lacquer base coats and crack coats is paramount in achieving desired results. Higher mill thicknesses will result in more and larger crackle “islands.” Less mill thicknesses reduce crackle effect and will result in small crackle “islands.”
- Basecoats
Use consistent levels of gloss for the base coat. Using a high gloss base coat will allow the crackle coat to separate and crack easier, whereas a low gloss base coat will reduce the cracking characteristics of the crackle coat.
- Spray Environment
Extreme ambient air temperatures and humidity, both high and low, will adversely affect the final results. Base coats in this regard are not as problematic as crackle coats. Crackle coats should be sprayed in climatic conditions ranging from 70 to 85 degrees and up to 80% relative humidity. Crackle coats should be applied over the entire project at the same time and under the exact same climate conditions.
- Equipment
Attention must be paid to setting up equipment in a consistent manner. Atomizing air, fluid pressure, and flow rates must be determined and recorded and set up accordingly.
- Materials
Viscosity and temperature of the coating must be maintained consistently to assure satisfactory results.
Application of Crackle Finishes
When applying crackle coatings, I find air-assisted airless to be the most successful technology to use. The hydraulic pressure, along with atomizing air, will provide for a high degree of material atomization, allowing the finisher control over the spray application. I recommend using a medium size orifice opening with a wide angle spray pattern. The settings on the equipment should be moderately high on the hydraulic pressure due to the high viscosity of the material. Atomizing air should also be set on the high side to finalize the breakup of the material. Gun to part distance should range from 8 to 10 inches, but must always remain constant. Extreme care should be taken to ensure accurate triggering and consistent gun speed movement across the part. I recommend spraying one very light coat first with half-lapping to ensure an even coat across the entire part. Wait a very short period of time and respray the part with one full covering of wet coat spraying in the direction of the grain. If the crackle develops too stark a pattern, let the crackle dry for a few minutes and then lightly spray mist over the crackle to soften the appearance of the finish.
Crackle Systems
There are many ways to build a crackle finishing system. Clear coats or color coats can be used as base coats underneath the crackle coat. Pigmented crackle coats are most commonly used, although clear crackle is sometimes used over color base coats to provide subtle textures to the finish. Some crackle lacquer finishes, called “directional” crackle, may be brushed on. When applied artfully, these directional crackles will have a very authentic appearance to antique painted finishes that have been brushed on and have cracked over the years. When designing your crackle system, be sure to be creative and utilize glazes, shaders, and toners to create an endless array of effects.
Conclusion
As a general rule of thumb, it is not best procedure to apply a harder coating over a softer coating. Crackle lacquer does not provide a very high level of protective properties and requires an additional topcoat if used for kitchen cabinetry or other uses where extra durability is needed. According to most coating manufactures ,the recommended choices for finishes to apply over the crackle lacquer are vinyl sealer with conversion topcoat, conversion sealer with conversion topcoat, or two part urethane. We chose to test conversion sealer and topcoat systems from several manufactures following their procedural recommendations. Each system was extensively tested over a long period of time and all passed with acceptable results. Manufacturer’s coatings are not formulated exactly alike; therefore I always advise finishers not to deviate from strict adherence to manufacture’s recommendations regarding the use of their products in the system selected, and that all finishers should perform test panels to verify the recommendations.
Once you gain control over your crackle finishing process, enjoy crackling and watching the amazing effect of crackle “crazing” right before your eyes!
Sample Crackle Finishing Schedules
System 1
Step 1 Dark NGR dye stain over white wood
Step 2 Light application of clear gloss lacquer
Step 3 Scuff sand
Step 4 Full wet coat clear gloss lacquer
Step 5 Light coat contrasting color pigmented crackle lacquer followed by full wet coat
Step 6 Topcoat with clear precat lacquer
System 2
Step 1 Light application pigmented lacquer basecoat
Step 2 Scuff sand
Step 3 Second application full wet coat of pigmented lacquer basecoat
Step 4 Light coat contrasting color pigmented crackle lacquer followed by full wet coat
Step 5 Full wet coat conversion varnish sealer
Step 6 Scuff sand
Step 7 Wiping glaze
Step 8 Shader
Step 9 Conversion varnish topcoat