Coatings technology has advanced rapidly over the last couple of decades. Two big reasons for the advances in durability, more environmentally friendly, and applications are the chemistries of the coatings and the finish systems that the individual coatings are a part of.
Finish systems are specific coatings used together in a specific process to produce a specific result. As indicated in the previous blog, these finish systems are tested extensively by the manufacturer to ensure the properties advertised are reproducible when following the process. Changes to the finish system by the finisher can result in reduced properties of the entire finish system. The vast majority of finishers are not able to adequately test the full range of durability requirements due to a lack of testing equipment and knowledge of how to test and evaluate the results.
We will look at some problems that can happen when the finish system is varied. The first was prevalent in the 1990s as water based coatings were starting to be used more and many finishers had an interest in lowering their VOC emissions. Many finishers wanted to use water based coatings; however, the water-based coatings would cause grain raising as the first coat was applied to the wood. Many finishers wanted to use their normal solvent based sealer then use water based topcoats. This would eliminate the grain raising as the wood was sealed off with the solvent based sealer and the VOC emissions would still be noticeably reduced. The question finishers had was will the water based topcoat adhere to the solvent based sealer. The answer is yes and no. It really depended on the specific solvent based sealer the and specific water based topcoat. Some solvent based sealers would work with some water based topcoats. However, there was not much of a “blanket” statement that could be made where a specific solvent based sealer worked under all water based topcoats. And there was not a water based topcoat that worked over all solvent based sealers. Testing was necessary for each combination of solvent based sealer and water based topcoat. Not only adhesion testing is needed, but the full battery of tests required for the end use of the finish system. Finishers who took it upon themselves to create a finishing system combining these products, without the manufacturer’s recommendation, also took on the responsibility of the results of the finish system.
Another common problem is using all the specified coatings that a finish manufacturer recommends but changing the process. Even changing something as simple as the amount of dry and/or cure time can cause some properties of a finish system to fail prematurely. Reducing the amount of dry time has created many problems including premature cold check failure. A number of times finishers had problems with cracks showing up months, weeks, days, and even hours after finishing with an acid catalyzed system and needed to know what caused the problem to prevent it in the future. Often during failure analysis, it was discovered that the specific job with the cracking problem was rushed through the finish process to meet a deadline. Since the drying and/or curing time is often the majority of time in a multiple step finish process, it is what is sacrificed to meet the deadline. The cause was early coats of finish were recoated before enough of the solvent/water had evaporated from the finish. The solvent became trapped in the early coat. This keeps the early coat soft and slows the cross-linking of the resins. The final coat is exposed to the air and dries and cures quicker since its solvent/water can evaporate from the surface of the finish. This means the final coat cures harder than the early coat. Having a soft foundation over a harder topcoat typically causes problems with the most noticeable being finish cracking. Failure can happen in only hours if the finished parts are placed in an environment of different temperature and or humidity that causes the wood to expand or contract. Often occurs in winter when parts are loaded on a cold truck. When the parts are unloaded cracks show up. Even without changes in wood dimensions due to temperature and humidity, the finish can fail when the trapped solvents from the early coats finally migrate to the surface and evaporate. The early coat “shrinks” under the more cured topcoat often creating stress the cured topcoat can’t handle resulting in cracks.
Many of the most common finish problems are related to the finish system being changed in the field without fully testing the results. Contact your finish supplier to learn more about the recommended finish systems.