I know you are grumbling, and you are right — there is no such thing as a completely clean paint job (Fig 1). Still, we cannot surrender to dirt; we must do everything possible to keep dirt from getting in prior to painting.
If you were to look up paint defects from dirt, you would find that some consider it a paint preparation problem. Though dirt can be introduced during the paint and dry process, those things that cause dirt to be left on the paint job or deposited on the paint are many — and some of the routine steps to prevent contamination can be very easy to forget or skip when you are in a hurry. In fact, while I do my best to list as many tips as possible, surely someone reading will think, "He forgot…”
Dirt in the finish can come from dirt in the sealer, basecoat or clear. It could appear as dirt throughout the paint job, or in distinct areas, or random and scattered nibs. Though dirt is often easily identified, sometimes solvent popping, fine fisheye or pot-life shock can be mistaken for dirt. So let's look at these three and determine how to distinguish between them.
Solvent popping (Fig 2)
Solvent popping is blistering in the paint caused by solvents not allowed to escape from the undercoat, basecoat or clear before the next coat is applied. The condition can be aggravated by forced drying or uneven heating of the surface. Solvent popping can be distinguished from general dirt because it typically occurs on a complete panel. Dirt is less general, confined to a specific area or appearing as random nibs.
Fine fisheye (Fig 3)
General fisheye are craters and easily identified, but sometimes small fisheye can be mistaken for dirt or even solvent popping. A close look at the defect will reveal a crater where the coating has not flowed over the surface. The difference between this and dirt is that the dirt will often be in certain areas and not generally over the surface of the panel. Also, the crater is an identifying aspect of fisheye. Painters should use tools such as a magnifying glass to look at the paint surface closely.
Pot-life shock
You may not find this name in the common paint defects category; however, the condition is caused by using paint that has exceeded its pot life. New painters may believe that once the pot life has been exceeded, paint quickly turns into a densely catalyzed glob, like what you might be see with primer when you come back after lunch and realize you haven’t cleaned the primer gun. Paint catalyzes much more slowly, and the first step is some of the paint hardening while a vast amount remains liquid. If sprayed in this condition, the paint can look like dirt. This defect must be removed and the job repainted, because the coating has failed.
There are four times during the painting process when a painter must consider dirt contamination:
· During mixing or when preparing the paint for application;
· During vehicle preparation (sanding and cleaning of the vehicle);
· During application, when the sealer, basecoat and clearcoat are being applied; and
· During drying, when either air amplifiers are used or flash time is allowed between coats
Mixing
Mixing is more critical than many think, and often not enough care is taken during the mixing and reducing process. A clean work environment is critical. Notice in Figure 4 that paper has been used to cover the workbench and the back wall. This aids in cleanliness during mixing and reducing, and it is also easy to replace if (or more likely when) a spill occurs. Also note that the scale surface is clean, which is necessary for accurate mixing/reducing weight.
Before pouring, technicians should clean the tops of the toner cans so nothing drops into the mix. Also, all paint should be strained before application. With the popularity of disposable paint cups, the strainer is built into the cup; but if disposables are not used, all paint must be strained before applied. It is advisable to keep the mixing room as clean as possible, and the person mixing paint should wear a paint uniform to keep any dirt that might be on the body surface out of the paint.
Preparation
Preparation includes sanding, cleaning, masking, bagging and preparing the booth and painter for paint application. This is a big step and a very critical one, as well. Sanding the undercoating and preparing it for painting, one could say, is the biggest and most important part of painting. We all like pulling the trigger and watching the paint flow on, but if the surface is not clean, free of debris and smooth, the paint job won't look good.
One of the key parts of all these jobs is inspection before you go to the next step. Is the surface sanded to the manufacturer’s recommended grit, and have you checked that no spots have been missed? Here is where a guide coat can come in handy (Fig 5). It not only shows scratches, but any other defects are seen as well.
Then the surface should be cleaned with a wax and grease remover. Don’t forget to clean inside the jams, where dirt may have migrated (Fig 6). Once the vehicle is completely cleaned, it is ready for masking. Vehicles should be masked from the inside out, and the tape should be rubbed down. Try taking a piece of tape and putting it down on glass; then take another and firmly rub it down. You can see through the glass that the tape that has not been rubbed down will not have the edges pressed firmly against the glass (Fig 7), and paint may migrate through the edges. The car should be masked outside the booth with the booth doors closed, so any dust that may be in the shop won't get into the booth and, later, onto the vehicle.
Preparation of the booth is another often neglected area. The paint booth should be the cleanest place in the shop. The outer doors should be closed at all times, only opened to take a vehicle in and out. Before spraying, the walls should be blown off, with the fans on, so any dust is drawn into the filters. The filters should be changed often, and the floors swept (with a fine broom used in the booth only) and mopped before each paint job.
An old-school method is to wet the floor before painting a vehicle. There are some problems with this, however. Water that splashes onto the vehicle and the high humidity in the booth can cause problems with both waterborne and solvent; but most of all, repeated wetting tends to rust the bottom of the booth prematurely.
Some painters chose to cover the floor for easy cleaning (Fig 8). There are coverings for the walls, as well, which will trap the dirt and prevent it from spreading onto the vehicle (Fig 9). When the coverings become loaded with dirt they are removed and new are applied. This speeds up the booth maintenance greatly!
Masking should likewise be done outside the booth so that dirt will not be allowed into the clean area. Then the vehicle should be blown off; the technician should pay special attention to areas that may trap dirt such as in door gaps and wheel wells. With the vehicle in the booth, any final masking, such as finishing the driver's door or bagging the vehicle with plastic wrap, should be done. Then the surface to be painted should get its final clean with the recommended wax and grease remover. The last dirt prevention detail before painting is to tack off, using the two-tack cloth method. (Many painters consider this tacking to be the first part of dirt prevention during application.)
During application, one of the most important tools used to keep a paint job clean is the painters’ suit. Yes, the suit is necessary to protect the operator from dangerous paint chemicals, but it is also extremely useful in keeping the job clean. Some speculate that upwards of 80 percent to 90 percent of the dirt that finds its way into paint work comes from the operator. Though this estimate may be high, it is safe to say that caution should be taken. If the suit is not new, it should be blown off (outside the booth) before being put on. If it is reusable or washable, it should be laundered often. Painters should blow themselves off as well before the suit is put on. Once the operator is covered, it is time to balance the booth.
Each time the booth door is opened, dirt can get into the booth and onto the vehicle. To make sure that this potential for contamination is keep to a minimum, the booth should be balanced. Most current booths have two air movement systems, one to blow clean air into the booth and another to move air out of the booth. If the booth is set so that there is a negative pressure inside the booth each time the door is opened, air from the shop will be drawn into the booth. Because air that is brought into the booth by its fan system is filtered, shop air can and will bring dirt in with it. To combat this, set the booth in a slightly positive mode as measured by a meter. The air being drawn out is slightly less than the air being pushed in, so when the door is opened, no dirty shop air is drawn in. Most clean painters make as few trips into the booth as possible and deny or limit others from coming in while painting is in progress. Some painters even mix the sealer, basecoat and clear, taking them in at the same time to keep the paint environment cleaner.
Once in, the painter will tack the vehicle, again using two tack cloths. A new one is used to tack off the area to be painted, wiping in one direction as if you are brushing off crumbs. This is an excellent time to do a final inspection of the prepped areas for anything that may have been missed. The second cloth (a used one is often used here) will tack off the remaining parts of the vehicle, including all the masked areas. Wipe in one direction as before, but away from the area to be painted. The last thing to be tacked is the paint hose. After all is set and the painter is about to start the job, use the second tack cloth to wipe the paint hose, so any dirt that may have been picked up by the hose as it was not in use will be removed. Remember that if the hose is disconnected and dropped to the floor for any reason, it should be tacked again.
Nibbing out defects before clear
Though this is a during-application step, it warrants special discussion. Though not all paint companies recommend de-nibbing, you should check with their recommendations. If a dirt nib or any imperfection is found at either the sealer or basecoat level, it should be removed before going on to the next layer of coating. Dirt nibs in sealer, if not removed before base is applied, will show as spots when detailed. Also, if the basecoat has dirt or other imperfections, they should be corrected now. Follow the paint maker’s recommendation, such as sanding lightly with P1000 and re-applying the coating; or some may recommend that the coating be "washed off" and reapplied. The defect must be corrected before another layer is applied. If all the precautions mentioned earlier have been observed, defects and nibs can be eliminated or at least kept to a minimum.
During drying
Most of the dangers of getting dirt into the paint job have been addressed, though during drying the paint is still vulnerable. Take everything out of the booth before baking, and keep the doors shut. Do not allow anyone, especially someone not wearing a paint suit, into the booth until it the paint surface is completely cured. When the bake cycle is over, the paint is no longer in jeopardy of dirt being deposited in the clearcoat.
Conclusion
I’m sure some of you are thinking that this is a long list of tasks that will surely slow you down. And when these tasks are laid out in an operating procedure, it may seem that way. But when put into practice and everything is in its place, very little time is added to the painting process. If many, most, or all the dirt nibs are eliminated, think of the time savings to be gained when less buffing is needed.
I once attended a buffing training session where the instructor said that if there are more than 8 to 10 nibs in a panel, the complete panel should be cut and buffed to avoid nib craters or texture differences! So shouldn't we strive to keep them to a minimum? As we determined earlier, we may not be able to have a perfectly clean paint job, but we should still strive for it.
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