Speeding up paint prep

Feb. 14, 2014
Using the right techniques can save time and improve the final product.
Figure 1

I once heard a new paint technician tell his friend that he loved painting, but he hated sanding. Well, I don't think that any of us would list sanding as the task that we most look forward to when working, but we must also face the fact that if a good paint job is 90 percent prep, then sanding is one of the most important jobs done when painting a vehicle. In fact, sanding is 30 percent of paint labor (Fig 1). That said, like everything else in the collision industry, the faster you can get it done, the more profit that can be made. Let’s take a look at some techniques that help speed the sanding process.

Washing and cleaning
One of the first steps when prepping a vehicle for paint is washing (with soap and water) and cleaning (with wax and grease remover). This important two-prong step will prevent a great deal of problems that may crop up later in the process. I say may because some painters might get away with painting a vehicle that is not clean, or they just may not recognize that some of their problems stem from not properly cleaning the vehicle before painting. Besides the obvious fish-eye problem, other problems such as dirt in the paint, missed defects and poor adhesion can all be traced back to a poorly cleaned vehicle. Some look at this step a waste of time, while others just overlook the importance of the process. But painting over a vehicle that is not properly cleaned only leads to problems later in the process.

Figure 2

Both “prongs” of this procedure are critical. In the soap and water step, pH balanced soap is used to remove contaminants over the entire vehicle, including the door jams, under the hood and the deck lid. Once the entire vehicle is soap-and-water cleaned, next the technician should clean the area to be painted with a wax and grease remover that is compliant with your shop’s paint system and the regulations of your area. This cleaning removes the contaminants that were not soluble by soap and water. The vehicle can be blown dry following the cleaning process so that the next step, sanding, can start immediately. A hand-held air amplifier that is used for waterborne paint drying works very well to dry the vehicle (Fig 2).

Inspection
Inspecting the area to be painted is vital to find any defect that must be corrected before initial prep of the paint area. This inspection can often be done while either washing or cleaning the vehicle. Look for things that may have been missed by the tech doing the body work, as well as for any defects that the estimator missed. If any are found, notify the estimator so a supplement can be written. By doing an initial inspection while washing the vehicle, the technician can make a repair plan at that time. During the inspection process, the tech should also retrieve the paint code and write it down on the work order, if it is not already there. Having the paint code early in the process and checking that all the needed supplies are available for the job will eliminate wasted down time later on. If some supplies must be ordered, the job can proceed with the prep while the order is shipped in. Before we start sanding, let’s first look at the way abrasives are graded so we can choose the correct one.

Figure 3

Abrasives
One of the more confusing changes in abrasives happened some years ago, but is still not fully understood by many technicians. This change is in the grading system of abrasives. For years, there was only one method of classifying abrasives; this was the CAMI or Coated Abrasives Manufacturer’s Institute system. This system allows larger percentages of grits other than the one labeled to be on the sandpaper. For example, a piece of 400 grit paper could contain some 360 grit and 500 grit particles and still be labeled as 400 grit. 

A second system is the FEPA — or Federation of European Producers Association — grading system. This European system, which uses a designator of P before the grit number, has a much tighter tolerance regarding grit sizes. The P (FEPA) papers have become the grit system that is most often called for in paint makers’ recommendations. Shop personnel should read, understand and follow recommendations by the paint system used in selecting grits.  

Actual grit sizes in the US grading system (CAMI) and the European system (FEPA) are similar up to about 220, but from that point on they vary greatly. For example, CAMI 240 grit is equal to FEPA 280, and CAMI 600 is equal to FEPA 1200. To make things even more confusing, there are two additional sandpaper grit systems: the Japanese Grading System (JIS), which does not use a designator, and the Micron grading system that uses a µ symbol following the grit number.

All of these grading systems can be confusing, and if there is no chart handy to tell which one is the correct grit for a particular task, it is best to follow the paint maker’s recommendations.

Figure 4

When a shop indiscriminately orders 400 grit paper, its technicians may not be getting the correct paper for the job at hand. Therefore, if the recommendation is to use P500 to P600 for final sanding to get the desired results, the shop should be careful to purchase P-rated paper for the paint technician to use.

The recommended method of sanding should also be followed. In fact, many paint manufacturers recommend a different grit when dry sanding by hand than when a technician chooses to wet sand. Some also have different machine sanding recommendations for wet sanding than for dry. As an example, the recommendation may be P 600-800 grit for wet sanding by hand, and P 400-500 for dry sanding by hand. When using a machine, the recommendation may be wet P 500-600 grit, and dry P 400-500 grit. As machine sanding is the more common method of sanding, technicians should be careful not to assume that the sandpaper grit they use when hand sanding will be the same as when they machine sand.

A discussion about sanding and abrasives is not complete without talking about a prep cart (Fig 3). Rolling up to the job with a prep cart containing everything that is needed to complete the job significantly reduces the time needed. The cart should be stocked at the start of the day with all the needed sandpapers, scuff pads, sanding paste, tape, etc.

Figure 5

Blocking: Power vs. hand
Ok, I was one of those guys you will see on the Internet “doing it my way.” I will admit that this one took me awhile to try and accept. For years I thought that those who tried to block with a DA were technicians that were less concerned about a quality finish. I believed that it was impossible to make a non-wavy, smooth, flat finish without blocking by hand — wet. In fact, in cases when I was super-concerned about getting a show quality finish, I would paint-stick (Fig 4) block the primer with a sprayed-on guide coat. This may have still been the case today, if some significant changes had not come about, such as better DAs. Even in the face of all the articles online stating that you can only get a flat blocking by hand, I now respectfully disagree.

Hand blocking with sprayed-on guide coat, either wet or dry, has been replaced by use of a painter’s DA's (a DA with a circular pattern that is best suited to power blocking). Interface pads, which should be used with grits of P400 or greater, help with sanding control. Also dry guide coat, which doesn't clog the finer grit paper as fast, should be used.  Yes, there is a learning curve, and you may not get the result that you're looking for the first time you try it; but with a little practice it can be achieved.

Figure 6

The key points are that first you use guide coat, even though you have been sanding for a long time and don't think you need it. The guide coat will help you see when to stop sanding. One of the most common mistakes made is sanding more than needed. The guide coat shows you just when to stop — do not continue to sand after all the guide coat had been removed. If you do, it turns the surface into a wavy mess.

Second, another common mistake is not holding the DA flat to the surface. The better you get at holding the DA flat to the surface you are sanding and not topping over the edge or riding over body lines, the better you will become at power blocking. Also, using a fine-finish DA pad and a finish DA with the correct size pattern is essential.

Lastly, recognize that the technique for sanding is also important. The surface should be sanded completely in one direction, such as left to right, and then in the opposite direction. When the entire surface has been sanded in both directions, check the surface. If more sanding is needed, repeat the process. The tool should be started ON the work surface and stopped with the tool OFF the work surface to keep from leaving marks. The sanding should be done with light operator hand pressure on the tool. For those who prefer, there are 3" by 4" orbital sanders that may be used to help in the blocking process.

DA types
DAs have for some time come with different handles, with different sanding pattern sizes, and different pad sizes and shapes. Though they may all look similar, they can be very different.

To help on DA pattern size:

3/32 DA =         3/32
1/8 = 2/16 =      4/32
3/16 =               6/32
5/16 =               10/32
3/8 = 6/16 =      12/32

Figure 7

Handles
The older DA with the handle that came down from the body (Fig 5) is still a very good tool when used for the correct job. Many body men prefer this type of DA handle because it is very aggressive. It is great when stripping paint and sanding plastic body filler.

However, for better control when finish sanding, the palm DA is much superior. The palm DA can be held flatter (with the non-palm sanders it is more difficult to keep the DA’s surface even), and it is also easier to control the amount of pressure on the tool, which should be light and steady.

Sanding pattern
DAs now come in a staggering array of different sanding patterns such as 3/32, 1/8, 3/16, 5/16, and 3/8. Which is best for a painter? The smaller pattern will leave a smoother finish than the more aggressive one, but it will take longer than the more aggressive pattern. Many painters have two Das — they use one for blocking, say a 1/8 (4/32) or a 3/16 (6/32). As well, they may have a very fine 3/32 for sanding with very fine paper (P1000 to P5000) when preparing for buffing, but that is another subject, to look into another time.

So if you have a DA now and it is not marked, how do you find out what size it is? You can test the pattern size by placing a piece of paper on it and lightly touching a pencil to it as it runs freely (Fig 6). DAs made specifically for painters have a smoother operating path and tighter pattern, and are suited for power prep, or dry blocking. Also, as the environmental laws change and more areas are mandating the removal of drains in shops, wet sanding may become more difficult. Thus dry sanding with vacuum pickup or sanding in prep decks may become a paint shop’s only option.

Figure 8

Interface pads
These are pads that fit between the DA pad (Fig 7) and the paper; they also make the cutting action of the DA less aggressive over high crowns. If a technician does not use pads like these, it is more likely that a cut-through over the high crowns will result. With the use of “interface pads,” as 3M calls theirs, the surface of the sandpaper is cooler, which extends the life of the pad, especially on finer grit paper. With the use of a guide coat, a DA and pad quickly sands smooth areas over featheredged surfaces, as well as over body filler. If imperfections such as sand scratches or contour mapping are present, they can be quickly seen, and the continued blocking can be concentrated on the area until it is corrected. Following the first sanding, a second guide coat can be applied and a finer sanding can quickly be done to assure that all scratches are removed and the proper grit finish is obtained.

Figure 9

Guide coat
Guide coats have been used for many years, but weren’t always. Even when an aerosol spray can was used, it needed time to dry and would load up the sandpaper quickly; thus, many technicians didn’t use it. Dry guide coat, though, can be quickly applied and immediately sanded with no appreciable increase of paper loading. Its new convenience virtually eliminates any reason not to use it (Fig 8). Guide coats quickly and visually reveal any imperfections in the repair, so even a novice technician can know when the surface is properly prepared (Fig 9). 

Good planning eliminates wasted time and materials, and can virtually eliminate paint flaws due to poor prep. We can produce the needed flawless surface in a shorter amount of time with the use of rolled primer, finish DA, dry guide coat, and fast and clean application of glaze. In fact, if the body technicians either significantly reduce or eliminate pinholes by mixing body filler without air bubbles and use dry guide coat when sanding body filler (thus making it flatter and smoother), glaze may not be needed as much in the paint department to prepare the surface for refinishing.

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