Paint Preparation: Categories And Procedures

Paint Preparation: Categories And Procedures
by Bret Sullins *
Paint preparation may well be the most important production step in the collision repair process. It’s also an area that is often taken for granted. Obviously, paint preparation does not generate the dollars that other production areas do, so let me explain why the success of a repair can hinge on proper paint preparation.
Being a shop owner, I have noticed that customers are the most critical when it comes to the look of the final refinish. It’s the most visible part of a repair and the easiest for a consumer to judge. A customer may not be able to tell whether a vehicle’s structure was properly restored, but most can spot a poor color match or uneven gloss.
At a recent seminar I attended on shop profitability, we were told the paint shop accounts for 20 percent of the gross sales, and that more than 75 percent of the work generated in the paint shop is made up of preparation steps. In many shops, the paint preparation steps are carried out by "low-tech" people who make the least pay. However, the job they are doing is critical — it accounts for the majority of the operations in the paint shop, and more importantly, it can bring success or failure to the final refinish. You could have the best painter in the world, but without proper preparation of the vehicle, that painter could end up looking real bad. Keeping this in mind, let’s take a look at some considerations and procedures for preparation that will make a painter’s work look its best.
Categories And Washing
Paint preparation can be broken down by the three categories of panels to be worked on: new panels, repaired panels and blend panels. There are also two subcategories in panels: metal panels and plastic panels.
One thing that’s true about preparing all panels is that they need to be washed. I know we have all heard this many times over, and I must confess that this doesn’t always happen at my shop. However, many problems could be avoided if this step was not overlooked, and in fact, was practiced religiously. Thorough washing is critical because there are many contaminants that will not come off without a good soap and water wash. This will also make for a cleaner job and workplace.
New Panels
Almost all new metal panels will come with an electro-deposited primer (EDP). The most notable exception to this is a galvanized part. New plastic panels could come either with or without primer. The inside edge of most metal panels will have to be painted. All paint manufacturers typically require the application of a coat of sealer to a panel before the topcoat is applied. The most common method of preparing the edge of a new panel for the sealer is to scuff it with a red scuff pad. (Note: there is at least one paint manufacturer that has a sealer that will bond to an EDP without sanding.)
I recommend that the outside of the panel be sanded with a rubber block and no rougher than 320 grit dry paper. I know a lot of shops use some sort of machine sanding, which is fine, though I also recommend no rougher than a 320 grit paper when using a machine.
The reason I prefer hand sanding when preparing a panel for sealer is because you can find imperfections easier due to a block being more accurate. Granted, a machine is much faster, but for a scuff sanding, doing it by hand will reveal any minor imperfections on a panel’s surface. Many times, a panel that was machine sanded ends up in the spray booth and the painter applies sealer to it only to then find a slight indentation or depression that could have been discovered earlier if it had been hand scuffed with a block.
Some may prefer to prime a new part, in which case a 180 to 240 grit machine sanding will give the primer a good "tooth" to bond to. However, a 320 grit paper is best if you are going straight to the booth, since the sealer will not have a problem filling that size of a scratch. In the case of the galvanized part or bare metal, you could sand with 240 grit paper. This should be primed with an etching primer for proper adhesion. I like to wash the panel again with soap followed with a solvent cleaning before spraying the panel.
Plastic Panels
In working with a new plastic panel, it is first necessary to determine if it is primed or not. Look for signs of primer overspray on the interior side of the panel, or sand in an area that is hidden to see if you cut through to raw plastic.
Whether a plastic part is primed or not, I recommend scuffing the part with a grey scuff pad (grey pads have a finer surface than red pads). You should never use a red scuff pad on a new plastic panel due to the rough scratch and possible gouging of the plastic. In conjunction with a pad, try using one of the cleaning products that is a soap, which also functions as a lubricant. This combination will put a good scratch into the panel. Using this same pad, also clean the inside of the part to remove any contaminants left behind from the manufacturing process. Plastic parts usually have some trace silicone left behind during the mold process, and silicone is the major offender that causes fisheye problems. It would be a good idea to bring the plastic to a temperature of 140 degrees Fahrenheit if it is unprimed to drive out any contaminants in the pores. After scuffing the part, you should follow with a plastic cleaner, preferably using a new scuff pad. At this point all you need is a final wipe and dry with a plastic cleaner. If the panel is unprimed, you will need to apply an adhesion primer for plastic. If the panel is primed, a plasticized sealer or a sealer that is flexible should be applied before spraying the colorcoat.
Repaired Panels
A repaired panel presents its own set of problems. Always consider if the panel being worked on is a previous repair or original equipment (OE). When dealing with a previously repaired panel, you have to consider whether the finish is a sound one. A solvent-soaked rag laid against the featheredged paint will reveal this. If the solvent attacks immediately by softening the finish, you will need to lightly apply your primer. By this, I mean that several thin coats with long flash times are better than thick, multiple coats applied with minimal flash times. Wash the panel and also inspect it for any other damage while doing this. You will first featheredge the repair with a 180 grit machine sanding followed by a final machine sanding with 320 grit paper where the primer will end up past the repair. A scuff pad is not the best for the area surrounding the repair since it will not give the primer sufficient tooth to adhere to.
Priming
Before priming, an appropriate solvent wash should be done. The type of primer used will depend upon the amount of fill that you need. Achieving proper fill is probably one of the most overlooked areas in the paint shop. What can commonly happen is that a helper is priming a vehicle and sees a lot of scratches and pinholes and thinks that a couple more coats will do. However, what is really needed is that the repair should be finished smoother or a different primer used that is designed for heavier filling.
One note here — primers by design are made to cure quickly, therefore always use the slowest hardener/solvent to avoid solvent entrapment. On bare metal patches, make sure you use an etching primer. Once the primer has cured, apply a guide coat and block sand the primer. Either hand or machine sanding is fine. I recommend cutting with 240 grit paper to quickly level the surface, followed by 320 grit. Depending on whether or not you will seal the panel, you may want to finish it in 600 grit. With a base/clear job, it’s preferable to finish sanding with 600 grit paper. Then follow this with a good washing, followed by a final solvent wipe before spraying the topcoat.
On a plastic panel, the general priming steps would be the same. The exception would be when you are dealing with a raw, unprimed plastic, which you would then treat as explained earlier. You would also use a plasticized primer or one designed for plastic panels.
Blend Panels
The proper preparation of blend panels is a key part to the success of many repairs. I have seen many peeling blends and rough scratches show through because of improper preparation. There are several cleaning products on the market that are designed for blend panels. These products are usually soap-based, some have an abrasiveness, as well. These are the same products that were discussed earlier for use in cleaning a plastic panel.
If the car is showing some fading, I recommend compounding to bring back the gloss. Using a grey scuff pad, thoroughly scrub the panel until it is completely dull. I would never recommend a red scuff pad because the scratch can be too rough, causing a distortion of the pearl/metallic flake when sprayed. You may want to lightly sand with 600 grit paper on the area where the paint will land. You should be careful not to sand out too far, as this can also cause distortion in the flake. After the final rinse, a solvent wipe and dry is needed and then the repair is ready to spray.
I have read many articles in the trade press on the subject of proper paint preparation, and I cannot remember one article that did not provide at least some further insight into the subject. Hopefully this article has done the same.
Proper paint preparation is a subject that requires "hands on" practice to fully appreciate the needed techniques. However, thinking of preparation by the category of panels to be dealt with and this review of general procedures has hopefully increased your knowledge of what is needed. For certain, paint preparation is an important consideration in a collision repair shop — if it’s done right, things will go smoothly, but if it’s not given the attention it deserves, the most noticeable part of a repair can go bad very quickly.
* Bret Sullins is owner of Eurasia Collision Repair in Fort Worth, Texas, and is a member of the Automotive Service Association (ASA). Sullins has also been a paint sales representative and a technical consultant for a paint company.
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AutoInc. Magazine ®, Vol. XLIV No. 6, June 1996