Your best paint jobs start with your best prep jobs.
Some surface prep techniques and recent changes are controversial. Not all technicians agree on the value of changes such as fine or "painter's" dual action (DA) sanders, use of guide coats, hand vs. power (DA) blocking and wet vs. dry sanding. Others are crucial steps for quality surface preparation that may not be getting the attention that they should, such as washing and cleaning, open bare metal time and bare metal treatment.
Because sanding is such a large part of labor cost, and prep sanding is such a large part of all sanding, the more efficient a shop can become will significantly influence a shop's overall profits.
Dual action sanders
Over the past few years DA sanders have changed. Originally, DAs had aggressive random orbital sanding patterns of 3/16 of an inch, specifically designed to remove paint coatings with course grit sandpaper such as P36 and to do featheredging with the finest grit paper being P350 at most. They were designed to be used at a 10-degree angle, which, because the whole sanding disk didn't hit the surface, made the tool very aggressive.
Older tools sold with long handles made them even more aggressive as the technician pressed down while tilting the pad even more than 10 degrees. Because the tool was used in this aggressive manner, it gained a reputation of not making the surface flat and straight enough for finish work. The first innovation was the development of a "palm" DA without the handle, which forced the operator to hold the tool flatter, making the resulting sanded surface flatter and smoother.
A few years ago, DAs became available with a finer sanding pattern (3/32 of an inch). These models, sometimes called "painter" DAs, were much less aggressive, and when mated with finer sandpaper such as P 400 and held flat to the surface, they were capable of producing a flat and straight surface rapidly.Guide coat
Though the guide coat technique has been around for a long time, not all technicians use it. Some believe that only "newbies" without the experience to feel imperfections in the surface need it. Others who don't use a guide coat claim that the black spray paint clogs the sandpaper, making the job slower, which can be true if not given proper dry time. However, using a guide coat quickly lets technicians see just how much more sanding is needed as repair work is being done. More importantly, it shows when to stop sanding.
Guide coats are helpful when blocking primer and can be helpful when sanding plastic body filler. Used with filler or polyester filler, guide coats will show the imperfections as the sanding is being performed, reducing the need to stop sanding to feel the surface as often. The use of a guide coat does not entirely eliminate the need for checking the surface by hand and for close visual inspection, but it markedly reduces the number of times needed to stop and check.Hand vs. power DA blocking
If a number of technicians were asked which was better, hand blocking or DA blocking, it is likely that a heated discussion would ensue. Many are of the belief that the only way to get an undetectable repair is to hand block primed repair areas. In fact, some would say that the only way that quality blocking can be accomplished is with paint stick blocking. But with the development of new and better DAs, better pads and better, longer-lasting sandpaper, dry primer blocking with a DA cannot only produce an undetectable repair, but can do it faster and thus result in a more profitable repair. Some manufacturers claim that dry primer sanding can be as much as 75 percent faster than wet sanding with less mess. (Still, dry sanding produces a significant amount of dust, which is best controlled with vacuum dust extraction equipment.)Some manufacturers offer a soft pad to be used between the DA pad and higher grit sandpaper. This pad not only provides a cushion, reducing pressure as the tool goes over high crown areas and reducing the likelihood of cutting through the primer; it also helps cool the surface of the sandpaper, making the paper last longer and clog less than paper that does not use this pad. Because this pad reduces cut-through, it is often used with P500 to P800 grit paper to prep blend areas or new parts for edge in.
Dry vs. wet
Dry vs. wet sanding also is a topic that will produce passionate discussion between painters. Some, who have been in the business for a long time, have stood in water or at least a wet area for years. When sanding was done with paper that was no finer than 400 grit, having a bucket of water and a sponge to squeeze a continuous stream of water over the area that was being sanded was the norm. As sandpaper became finer, though, dirty water could deposit debris onto the work and leave scratches instead of taking them out. To solve that problem, technicians changed to squirt bottles of clean soapy water to clean away the debris of wet sanding. Though this did a good job, it was time-consuming and messy. Even though the sanding sludge washed away after sanding, if allowed to dry, it could be difficult to clean off the surface properly. Most vehicles that underwent wet sanding were soap and water washed to remove sludge after sanding.In contrast, dry sanding debris can be vacuumed away while the vehicle is being worked on, and upon completion only a short cleanup period with compressed air completes the job. While it is typically recommended that the technician chemically clean the area to be painted after sanding, the task can usually be done without moving the vehicle to a wet area for washing.
Washing and cleaning
Two areas that deserve revisiting are washing and cleaning. While most prep techs and painters will tell you that the first step when prepping a vehicle for paint is soap and water wash (a common question on ASE test), washing is often not done before prepping is started. They think, "The car was washed before the body techs started on it." But clearly, as the body techs work on the vehicle, dirt from their gloved hands and other contaminants are re-deposited on the vehicle during normal repairs. If these are not cleaned off, the act of sanding will drive contaminants into the substrate, which could cause problems during painting. Both soap and water washing and chemical cleaning should be done to every vehicle as the first step in preparation.
You should perform a soap-and-water wash with a PH-balanced automotive soap in clean water and clean bucket. (Be aware that dish soap is not PH balanced.) In fact, the two-bucket method is best, with one bucket filled with clean, clear water and the second prepared with warm automotive soapy water. After rinsing with water (soft mineral free works best), the technician should wash the vehicle with soapy water from the soap bucket, cleaning the wash mitt in the clear water since it becomes dirty, before getting more soapy water to clean more of the vehicle.Using this method, the dirty wash mitt does not contaminate the soapy water with debris. After the vehicle is washed, it is then rinsed with plenty of clear water. The complete vehicle should be soap and water washed — even those areas that will not be painted.
Cleaning with wax and grease remover on the panels to be painted is next. Debris and contamination that interferes with the painting process may not be water-soluble; therefore, panels to be painted are cleaned with a chemical cleaner. This second cleaning removes all the likely contamination that remains.
Solvent cleaners may, in the near future, be eliminated by regulations in some parts of the country. The progressive shop may be wise to find a waterborne substitute that does a good job before one is mandated.
Bare metal cleaning
In the minds of most technicians, bare metal treatment pertains to the application of corrosion protection coatings such as acid-etch or epoxy primers. This important step is crucial for a professional, long lasting repair. What often escapes the attention of prep technicians is the recommendation that bare metal surfaces should be primed within a short period after they were last sanded. Some typical recommendations are that bare metal, which has been open to air for more than one hour before corrosion protection material is applied, should be sanded again before priming. Often an area of a vehicle that is open is DA-sanded within the hour before masking and priming. If a vehicle sits overnight after sanding and before corrosion-resistance paint is applied, it should be lightly sanded before priming to remove any oxidation. Aluminum oxidizes much faster than steel, and its critical open time is less.Final thoughts
Surface preparation, though a subject that most technicians have covered time and time again, continues to change. New tools, better sandpaper, and even new techniques are introduced as companies are hard at work to bring to the market new and faster techniques. Not every new tool or technique will be right for each shop. As an example, rolled primer, used extensively in Europe has not largely been adopted in North America. For this reason, shops and technicians should develop alliances with their jobber or manufacturer representatives, who often leave samples with a shop to try. This often gives the shop or tech the opportunity to see if the new process will produce as advertised. Trying out guide coat with power blocking, to see just how much faster it may be for your shop, could prove rewarding.
From time-to-time technicians should look at their production sheets for the latest recommendations from manufacturers. Shops should encourage jobbers and manufacturer representatives to visit and demonstrate their new tools and techniques. Let's face it, if you produce faster and run more work through the shop, not only do you profit, but the vendor may as well.
