The entry-level position of parts prepper is often viewed as the lowest on the totem pole. But if the job isn’t done properly, quality suffers.Many customers seem to think that parts arrive ready to install on a vehicle, but in reality much work is needed before the part is ready to paint. With the exception of some bumpers and trim items, parts must be prepared properly for painting. Several steps are needed to ensure a long-lasting finish. Different materials may require specific steps, but it always begins with a clean surface. Keeping it clean
One of the most important refinishing steps is cleaning: it must be done several times during the prep work. Any contaminants from the technician’s hands such as dirt, lubricants or oil can cause problems with the finish. Typically, a solvent—a substance capable of dissolving another substance—is used to loosen the contaminants from the surface being cleaned. Technically, water is a solvent, although painters usually think of organic solvents such as lacquer thinner or wax and grease remover.Mechanical agitation with a sponge, cloth or brush may aid in loosening the grip the contaminants have on the surface. The contaminants are then in suspension or solution in the liquid. If they actually dissolved into the liquid, they are said to be in solution. If they are just floating in the liquid, they are in suspension. Sugar in iced tea provides a good example of both of these. The sugar easily dissolves into warm tea, but frequently remains only suspended in cold tea. Rinsing or drying the surface with a clean cloth removes the contaminants from the surface. Water spots result if water is not rinsed off of the car. The small particles in the dirty water remain on the surface if the water is allowed to evaporate instead of being rinsed and dried off. In some areas of the country, water has enough mineral content that even clean water will cause spotting. In prepping parts for painting, soap and water, plastic parts cleaners—comprised of mostly water with alcohol or ammonia—and wax and grease removers are commonly used. Safety concerns and VOC regulations dictate that stronger solvents should not be used for cleaning purposes. With any solvent that is to be wiped dry, work on small areas so the area being cleaned stays wet until wiped dry to keep contaminants in the liquid.How smooth is smooth?
Sanding is the next step in the process, and it needs more attention than many technicians give it. Abrasives are used to scratch the surface so primer will stick. They are also used to make the surface smooth enough so that the final coats of paint will shine. However, there is a fine balance between too smooth and not smooth enough at each step in the process. Some primers form a chemical bond with the surface, but most rely on mechanical grip for adhesion. So how smooth, or rough, the surface is can be critical to the performance of a paint system. The industry standard is for all body filler repair work to be sanded to 180 grit before priming. Some primers are aggressive enough that they can be applied over clean, unsanded E-Coats. The lesson here is that the specific instructions of the system being used must be followed. There are some basic guidelines that apply to all prep work, but deviation from the primer’s data sheet instructions may compromise the performance of the refinish system.Sandpaper is graded by grit size. There are different standards for sandpaper grits (see sidebar), and in the finer grits, 600 grit is about 1,200P grit. Substituting a Coated Abrasives Manufacturer’s Institute (CAMI)-rated sandpaper for a Federation of European Producers Association (FEPA)-rated sheet will produce very different results. Although abrasive paper in grits finer than 10,000 are used in some polishing work, the collision repair industry generally uses paper from 36 grit up to about 3,000 grit. In prep work, paper from 80 grit to 600 grit is common. Generally, paper with higher numbers, or finer grit, remove more material without scratching as deeply. For example, 80 grit paper makes a deep scratch, which must be sanded out with finer grits before priming. Paper with 180 grit has many more abrasive particles per square inch, but they are smaller, so it doesn’t scratch as deeply. An interesting experiment to learn more about grit quality is to sand down identical areas with several grits. Mark off 1 ft. by 1 ft. squares on a hood that is being replaced and sand down each square using a different grit size. Time each one to see which paper does more work per minute. This is also a good way to compare different brands of paper with identical grits. Generally, using the finest grit paper that will accomplish the task is the most efficient method. Primer is generally sanded with courser grits for blocking, when removing a lot of material quickly is important and then finer grits to make a smooth surface for the sealer, color and clear coats.Some surfaces only need to be “scuffed” with an abrasive pad. The pads are color-coded and marked “coarse,” “medium” and “fine.” They are roughly equivalent across brands, but there is not a grit number associated with them. Scuff pads may also be used with a liquid or paste abrasive when preparing surfaces for blending or cleaning plastic parts. These are relatively fine abrasives and work well on uneven surfaces or for light scuffing.Materials make a difference
The makeup of the part determines which procedures should be followed to prepare the surface for primers. Plastic parts require more attention to cleaning than metal parts. Even within the term plastic there are variables. Flexible plastic parts such as bumpers and outer panels that are not primed at the factory may have mold release compound on the surface. Soap and water followed by a plastic parts cleaner may do the job. If the part still feels a little slippery, wash it again. Heating bumpers is a technique that is popular with some repairers. The theory is to drive all the mold release compounds to the surface so they can be washed off. This is usually not necessary, but it may help with problem parts. While some paint manufacturers recommend it, others do not. Strong solvents must not be used on unprimed plastic. Many plastics absorb solvents, which will then prevent adequate adhesion. Plastic parts that arrive with primer on them should also be cleaned thoroughly, then scuffed and primed. Some of the primers used on plastic parts are solvent sensitive and will lift if you use a strong solvent-based primer. A waterborne primer may be used in these cases. Otherwise, use light coats so the solvents evaporate quickly before they can react.Composite parts contain reinforcing fibers and should also be cleaned and sanded. They tend to be somewhat porous, as are the repair products used on them, so strong solvents should be avoided, or heat should be used to force dry the part after cleaning. Because of the porosity and pinholes that may be present, particularly if the part has been repaired or bonded in place, rolling the primer in place can be quite effective. The physical pressure pushes the air out of the little holes and fills them with primer. Sprayed products tend to bridge over pinholes. Oftentimes, new aluminum parts and steel parts come primed. In this case, simply cleaning the surface and scuffing as recommended by the primer supplier will suffice. If bare metal is exposed, however, corrosion protection must be addressed first. Metal-treatment systems, self-etching primers or epoxy primers can be used on the bare metal. Make sure the primer chosen is intended for use with the metal. Some primers are rated for use over aluminum, while some are not. If the exposed metal areas are small, some filler primers can be used. Any primer with fillers in it has some porosity, so priming large areas of bare metal could result in moisture surfacing. Again, be sure to adhere to the paint manufacturer’s specific recommendations.Edging
Many parts need to be painted before they are installed, because some sections of the vehicle cannot be painted simultaneously with the outside panels. Edging, or cutting-in, takes care of this. This is frequently one of the prep person’s responsibilities. In fact, cutting-in parts is where many painters develop their basic spraygun skills. Care should be taken to make sure these internal painted surfaces receive the level of preparation as the outside panels. Internal finishes must also match the finish of other internal surfaces when the job is completed.Preparing the prepper Preparing a new part for the finishing steps is an important task often left to the beginners in our industry. Locating the prep area away from any source of contamination must not be overlooked when laying out the shop. Cleaning, sanding, proper selection, mixing and spraying of the primers is critical to the quality of the job. Selecting the right people and instilling in them the importance of their work is also just as important to overall quality.
One of the most important refinishing steps is cleaning: it must be done several times during the prep work. Any contaminants from the technician’s hands such as dirt, lubricants or oil can cause problems with the finish. Typically, a solvent—a substance capable of dissolving another substance—is used to loosen the contaminants from the surface being cleaned. Technically, water is a solvent, although painters usually think of organic solvents such as lacquer thinner or wax and grease remover.Mechanical agitation with a sponge, cloth or brush may aid in loosening the grip the contaminants have on the surface. The contaminants are then in suspension or solution in the liquid. If they actually dissolved into the liquid, they are said to be in solution. If they are just floating in the liquid, they are in suspension. Sugar in iced tea provides a good example of both of these. The sugar easily dissolves into warm tea, but frequently remains only suspended in cold tea. Rinsing or drying the surface with a clean cloth removes the contaminants from the surface. Water spots result if water is not rinsed off of the car. The small particles in the dirty water remain on the surface if the water is allowed to evaporate instead of being rinsed and dried off. In some areas of the country, water has enough mineral content that even clean water will cause spotting. In prepping parts for painting, soap and water, plastic parts cleaners—comprised of mostly water with alcohol or ammonia—and wax and grease removers are commonly used. Safety concerns and VOC regulations dictate that stronger solvents should not be used for cleaning purposes. With any solvent that is to be wiped dry, work on small areas so the area being cleaned stays wet until wiped dry to keep contaminants in the liquid.How smooth is smooth?
Sanding is the next step in the process, and it needs more attention than many technicians give it. Abrasives are used to scratch the surface so primer will stick. They are also used to make the surface smooth enough so that the final coats of paint will shine. However, there is a fine balance between too smooth and not smooth enough at each step in the process. Some primers form a chemical bond with the surface, but most rely on mechanical grip for adhesion. So how smooth, or rough, the surface is can be critical to the performance of a paint system. The industry standard is for all body filler repair work to be sanded to 180 grit before priming. Some primers are aggressive enough that they can be applied over clean, unsanded E-Coats. The lesson here is that the specific instructions of the system being used must be followed. There are some basic guidelines that apply to all prep work, but deviation from the primer’s data sheet instructions may compromise the performance of the refinish system.Sandpaper is graded by grit size. There are different standards for sandpaper grits (see sidebar), and in the finer grits, 600 grit is about 1,200P grit. Substituting a Coated Abrasives Manufacturer’s Institute (CAMI)-rated sandpaper for a Federation of European Producers Association (FEPA)-rated sheet will produce very different results. Although abrasive paper in grits finer than 10,000 are used in some polishing work, the collision repair industry generally uses paper from 36 grit up to about 3,000 grit. In prep work, paper from 80 grit to 600 grit is common. Generally, paper with higher numbers, or finer grit, remove more material without scratching as deeply. For example, 80 grit paper makes a deep scratch, which must be sanded out with finer grits before priming. Paper with 180 grit has many more abrasive particles per square inch, but they are smaller, so it doesn’t scratch as deeply. An interesting experiment to learn more about grit quality is to sand down identical areas with several grits. Mark off 1 ft. by 1 ft. squares on a hood that is being replaced and sand down each square using a different grit size. Time each one to see which paper does more work per minute. This is also a good way to compare different brands of paper with identical grits. Generally, using the finest grit paper that will accomplish the task is the most efficient method. Primer is generally sanded with courser grits for blocking, when removing a lot of material quickly is important and then finer grits to make a smooth surface for the sealer, color and clear coats.Some surfaces only need to be “scuffed” with an abrasive pad. The pads are color-coded and marked “coarse,” “medium” and “fine.” They are roughly equivalent across brands, but there is not a grit number associated with them. Scuff pads may also be used with a liquid or paste abrasive when preparing surfaces for blending or cleaning plastic parts. These are relatively fine abrasives and work well on uneven surfaces or for light scuffing.Materials make a difference
The makeup of the part determines which procedures should be followed to prepare the surface for primers. Plastic parts require more attention to cleaning than metal parts. Even within the term plastic there are variables. Flexible plastic parts such as bumpers and outer panels that are not primed at the factory may have mold release compound on the surface. Soap and water followed by a plastic parts cleaner may do the job. If the part still feels a little slippery, wash it again. Heating bumpers is a technique that is popular with some repairers. The theory is to drive all the mold release compounds to the surface so they can be washed off. This is usually not necessary, but it may help with problem parts. While some paint manufacturers recommend it, others do not. Strong solvents must not be used on unprimed plastic. Many plastics absorb solvents, which will then prevent adequate adhesion. Plastic parts that arrive with primer on them should also be cleaned thoroughly, then scuffed and primed. Some of the primers used on plastic parts are solvent sensitive and will lift if you use a strong solvent-based primer. A waterborne primer may be used in these cases. Otherwise, use light coats so the solvents evaporate quickly before they can react.Composite parts contain reinforcing fibers and should also be cleaned and sanded. They tend to be somewhat porous, as are the repair products used on them, so strong solvents should be avoided, or heat should be used to force dry the part after cleaning. Because of the porosity and pinholes that may be present, particularly if the part has been repaired or bonded in place, rolling the primer in place can be quite effective. The physical pressure pushes the air out of the little holes and fills them with primer. Sprayed products tend to bridge over pinholes. Oftentimes, new aluminum parts and steel parts come primed. In this case, simply cleaning the surface and scuffing as recommended by the primer supplier will suffice. If bare metal is exposed, however, corrosion protection must be addressed first. Metal-treatment systems, self-etching primers or epoxy primers can be used on the bare metal. Make sure the primer chosen is intended for use with the metal. Some primers are rated for use over aluminum, while some are not. If the exposed metal areas are small, some filler primers can be used. Any primer with fillers in it has some porosity, so priming large areas of bare metal could result in moisture surfacing. Again, be sure to adhere to the paint manufacturer’s specific recommendations.Edging
Many parts need to be painted before they are installed, because some sections of the vehicle cannot be painted simultaneously with the outside panels. Edging, or cutting-in, takes care of this. This is frequently one of the prep person’s responsibilities. In fact, cutting-in parts is where many painters develop their basic spraygun skills. Care should be taken to make sure these internal painted surfaces receive the level of preparation as the outside panels. Internal finishes must also match the finish of other internal surfaces when the job is completed.Preparing the prepper Preparing a new part for the finishing steps is an important task often left to the beginners in our industry. Locating the prep area away from any source of contamination must not be overlooked when laying out the shop. Cleaning, sanding, proper selection, mixing and spraying of the primers is critical to the quality of the job. Selecting the right people and instilling in them the importance of their work is also just as important to overall quality.
