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| · Tech Tips · | 07.28.2006 |
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When Good Paint Jobs go Bad
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You've just completed a comprehensive body work project, sweated out the paint process and now you're ready to show your buddies... and there they are: Paint imperfections. Bad paint can ruin all the great work you've just done. One of the more common painting problems is solvent popping. Join us as we take a look at this pesky problem, and give you some tips on preventing it
What Is Solvent Popping?
Solvent popping is simply the occurrence of small blisters, or bubbles, in the paint surface that results from trapped solvents expanding and breaking through the surface of the paint film. This is illustrated in Figure 1. In the left-hand side of the illustration, we see the application of the new finish and the presence of the solvent in that film. Remember, the solvent is used as the medium, or carrier, of the paint to the surface of the vehicle body. And more importantly, that solvent, which is in a liquid state, must be removed from the paint in order for the paint to dry.
If the paint or primer is applied properly and the correct solvent is used, the solvent evaporates (changes from a liquid to a vapor state) out of the paint through the surface, allowing the paint to dry. When solvent popping occurs, the solvent becomes trapped in the new finish because the surface of the paint has skinned over. As the solvent evaporates, the gas bubbles rise to the surface of the paint, but have a tough time escaping. Those bubbles that do escape break through the surface, causing the identifiable blisters associated with this condition. This is shown in the right-hand side of Figure 1. |
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After the paint dries and cures, you're left with a surface that is covered with little pock marks that are about one millimeter in diameter. Not only can you see this damage, but you can feel it as well. Unfortunately, you can't simply sand down the damaged area and repaint it. Because the solvent is mixed with the paint, there is solvent throughout the entire vertical layer of the new finish and not all of the gaseous solvent escapes. The result is craters throughout the paint film. In other words, as you begin to sand out a damaged area and remove the craters visible at the surface, you'll probably encounter craters below the surface.
Something else that you'll see as you sand out the craters is an increase in the size of the bubble before the crater is removed. When the gas bubble of solvent finally breaks through the surface, the top part of the bubble breaks, leaving a larger diameter crater below the surface. It's like the tip of the iceberg - there's more below the surface.
For all of the reasons mentioned above, an area in which solvent popping has occurred must be sanded down to the old paint finish and repainted.
You have to start over from the beginning as though you hadn't painted the surface. Since solvent popping isn't a correctable condition, the best course of action is to prevent it from occurring in the first place.
The Causes of Solvent Popping
 So what causes solvent popping? We already mentioned the paint surface skinning over, trapping the gaseous solvent bubbles. But what brings on that condition? The most likely culprit is the thinner being used. In many cases, the use of a fast-drying thinner causes the surface to skin over before the solvent can evaporate out. The use of the proper thinner, or solvent, based on the spraying conditions, is critical (more on that later).
Another common cause of solvent popping is excessive film thickness. This results from applying heavy coats of paint, or not allowing sufficient drying time between coats. Either way, the paint film is too thick and the evaporating solvent can't escape.
Now that we've discussed what solvent popping actually is and the conditions that cause it, how do we prevent solvent popping from occurring? As with all the procedures associated with painting, attention to detail is paramount. At the top of the list when painting a repaired surface is the selection of the correct thinner. And this selection is based on the conditions in the area in which you are painting, which are affected by the weather conditions on the other side of the walls of your garage, paint booth, etc.
Preventing Solvent Popping
Now that we've discussed what solvent popping actually is and the conditions that cause it, how do we prevent solvent popping from occurring? As with all the procedures in the body shop, attention to detail is paramount. At the top of the list when painting a repaired surface is the selection of the correct thinner. And this selection is based on the conditions in your shop, which are affected by the weather conditions on the other side of the walls of your shop.
The two most important parameters are temperature and humidity, with temperature being number one. Ideally, we would all like to spray when the temperature is 70º F with a relative humidity of 45 to 55 percent (this will allow for normal drying time). How often do you have these conditions?
To compensate for conditions that are less than ideal, different solvents are used to control drying times. Not surprisingly, hot, dry conditions produce the fastest drying times (high humidity can present all kinds of problems), while cold, wet conditions produce the slowest drying times. It should be obvious that you have to keep a few different types of thinner in your garage, especially if you're in a part of the country where you experience all four of the seasons (the highest occurrence of solvent popping is in the spring and fall due to the variability of the weather at these times of the year).
As a general rule for solvent selection, use a slow-drying solvent for fast shop drying conditions (warm) and a fast-drying solvent for slow shop drying conditions (cold). And keep in mind that while different types of paints share common characteristics, there is variability from manufacturer to manufacturer. So, if you have any questions about selecting the proper solvent, contact the manufacturer directly.
Other recommendations for preventing solvent popping include the following:
1. Apply the paint in thin, wet films. Allow sufficient drying time between coats. Don't pile on or double wet coats.
2. When using a heat source, be sure to set that source at the correct temperature. Do not place the heat source too close to the new finish.
3. Allow for proper flash time between applied coats of paint, and allow each coat to flash naturally. Avoid fanning the new finish.
Solvent popping is a serious problem that can ruin an otherwise good paint job. The only recourse is to completely sand the affected area and repaint that area of the vehicle. Remember, the condition is easily preventable by paying attention to the details; namely, the conditions in your shop, solvent selection and spraying techniques. |
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