Here are tips for repairing damaged coatings effectively.
Checking and crazing are adverse conditions that usually appear later in the life of a coating and can be caused by fluctuations of temperature in the wood and the coating. In most cases, only the coating has the checking. The effects are seen as erratic crack lines running across the coating.
These usually are permanent conditions even after the coating has been restored; they may return again because the problem is in the movement of the wood, and the coatings always follow woodâs expansion and contraction. As the wood continues to increase and reduce the size of the coating, it will in time give out and end up cracking in different degrees.
There are other causes to these problems, which usually start when a project is first constructed and finished. Causes include: the moisture content in the wood, the shop temperature being out of the safety range for finishing, using the wrong solvent, not mixing the coating thoroughly throughout the entire finishing process or not evenly blending the plasticizers into the coating. All of these factors can and do contribute to the problem.
Understand Your Coatings
To help you understand a little more about coatings, there are basically two types. One is âevaporative,â which means the coatings will re-dissolve in similar solvents (shellac, nitrocellulose lacquer, acrylic, CAB, etc.). The other coatings are known as âreactive,â which means that once they are cured, they will not dissolve in their own solvents (poly, varnish, oil finishes, conversion varnish, polyester, pre-catalyzed lacquer, etc.).
|This sample shows a finish that is marred by checking.||The first step in making a repair is to clean the surface with mineral spirits.||The sanding process starts with 400-grit sandpaper.|
Checking and crazing will occur in both types of coatings. First sanding and then softening the check lines with certain solvents and padding agents can repair an evaporative coating, but it does take some skill to accomplish. A reactive coating is first sanded down to remove the check lines or crazing and then rubbed and polished to match the original sheen.
In some cases, depending on the original coating that was used, they may be coated over, too. However, all of these repairs are considered âcosmeticâ and should not be considered permanent. They are normally intended to make the piece look better.
The key to making repairs is in the sanding. First, you must remove the check lines without affecting the color in the stain, glaze or shading stain in the coating. If the color has to be sanded off, you may have to try coloring in the sand-through or strip the finish off.
If cracking can be sanded down without affecting the color, then you can continue the process. You will need sandpaper in the following grits â 400, 500, 600, 800, 1000, 1200, 1500 and 2000. Start with the 400 sandpaper to remove the checking and crazing lines, then use the 500 grit to remove the scratch pattern from the 400. Follow this process all the way through the sandpapers up to the 2000 grit.
At this point, the coating will be incredibly smooth. So if you decide to pad or French polish with either one of the padding mediums, or if you spray on a compatible coating, you will not see any sanding lines after you finish. This is especially important on glossy finishes, as the coarse sandpaper scratches will telegraph right through the coatings and ruin the repairs.
|It is important to follow through with each of the grits.||In some cases, you can compound up the sanded coatings to match the sheen.||The check lines have been removed and the coating is restored.|
â¢ Evaporative Coatings: Clean the surface with mineral spirits to remove any wax, polish, compound or other residue that remains on the piece. Do not use any kind of liquid if the cracks are through the coating, as this can create problems between the substrate and the coating.
Start by using a 400-w/d sandpaper to sand out the lines. Use extra care not to sand off any color that may be in the coating or any stain in the wood. Clean off the area and then either French polish or use a padding lacquer to pad the surface to build up the coating and the sheen to match the rest of the project. If you have a compatible coating, you can spray the repaired surface to match the overall sheen.
â¢ Reactive Coatings: Follow the same instructions as for evaporative coatings. However, in some cases you may be able to compound up the sanded coatings to match the same sheen. You also can use a compatible coating to bring back the coating and attain the same sheen. It is very important that you do not remove any color in the coating or the stain in the wood when you are sanding down the check lines.
It also is important that the coatings you select are compatible with both types of coatings. To test for the evaporative coatings, use either denatured alcohol, lacquer thinner or acetone; any one of these solvents will affect an evaporative coating. You can put a little solvent on a cloth and lightly push the cloth against the coating. If it leaves the telltale sign to show that the solvent softened the coating, then you will know for sure that it is an evaporative coating.
Try using the three same solvents on reactive coatings. It should not affect them at all.
It is important to know which coating you are working on. Evaporative coatings are much softer, they are easier to sand and will work with shellac, lacquer and some acrylics. Reactive coatings are much harder and more difficult to sand.
You might first try using a vinyl sealer as a barrier coat, then an acrylic, pre-cat or conversion varnish to recoat. Always do tests in an inconspicuous area on an evaporative coating before you try it on the actual checked area. Do complete tests, de-wax, sand out the lines and any other defects, then try padding with either padding lacquer or French polish. You should also try the sanding and the compounding process on a reactive coating.
If you are doing a complete restoration project, once you truly understand the complete coating repair process you can start working on the rest of the problems.
Mac Simmons is a freelance writer and 50-year veteran of the furniture finishing, refinishing and restoration trades. He has written articles for woodworking magazines in the U.S., U.K., Canada, Germany and Australia and just introduced a new finishing e-book on CD for $24.95. For information or orders, write Mac Simmons, Box 121, Massapequa, NY 11758. Other questions may be directed to him via e-mail c/o firstname.lastname@example.org.
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