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When Painting Two-Tones: Bag It

Great material savings can be achieved when high temperature-resistant bags are used for reverse masking in paint shops for getting two-tone paint jobs done. Here's how it is done.

Sometimes, two-tone paint jobs on light trucks like sport utilities are all the rage in the market. Unfortunately, they also often times tend to set people who are doing the accounting into a rage for the simple reason that they can be rather expensive.

Step 1
Step One: Here the vehicle has been painted with the primary color. Note two things: the blue fine-line tape that's been applied to mark where the second color will be painted and second, the roll of bags that's located just behind the cab. The bag is pulled off the roll just like a plastic bag in a fruit market.
Step 2
Step Two: A piece of high temperature masking film, 6-in. wide, is taped just above where the second color will be painted. About 1/4 in. of the fine line tape (which is 3/4-in. wide)remains visible. The purpose of the fine line tape is to create a good blend line between the two colors.
Step 3
Step Three - and beyond. Here the vehicle is snugly taped into the bag. The vehicle will be sent back through the paint booth for the second color and then right into the oven. This approach minimizes the amount of paint necessary to paint a two-tone vehicle.

As John D. Sheridan, Transportation Markets manager, 3M Industrial Tape and Specialties Div. (St. Paul, MN) explains, one of the standard operating procedures in paint shops facing the two-tone requirement is to:

  1. Paint the vehicle the secondary color. Bake.
  2. Mask the secondary color area with a polypropylene material.
  3. Put it through the paint booth again and paint the primary color.

The downside to this method is cost. Sheridan estimates that it costs about $280 for the paint on a pickup or sport utility vehicle. Not the labor and overhead. Just the paint. So with the standard procedure for doing the two-tone paint job, there's $280 times 2. And only a portion of each of those coats is visible.

Not surprisingly, 3M has an alternative approach, one that Sheridan calls a "reverse masking process." In this routine, the vehicle will still have to go through the paint booth twice. But the difference is that only a fraction of the area will be given the second coat of paint.

The way this works is that the vehicle is painted with the main color. After baking, there is a color-break line established with a fine line tape that's ¾-in. wide. Then, in effect, the vehicle is going to be placed in a giant bag, a bag that's somewhat analogous to those plastic bags that you find on rolls in fruit markets. Of course, these bags—or there are also sheets available—while they are on rolls, are certainly physically more robust than the kind you put your apples and oranges in.

For example, there is 7300 Antistatic OEM Paint Repair Bags and Sheeting, which is a soft polypropylene film that's capable of handling bake cycles of up to 310°F for one hour. The bags are custom sized to the vehicle being painted; the sheets are available with up to a maximum width of 89 in.

Fundamentally, the approach is to first take a piece of high-temperature masking film that's 6-in. wide and to tape it top and bottom so that there is about ¼ in. of the fine line tape exposed at the bottom. This serves as a transition. Then the bag is placed over the vehicle. It takes two people, one on either side of the vehicle, to put a bag on a vehicle. The antistatic property facilitates the unfolding of the bag. The bag (or sheet) is tucked and taped so that the material doesn't go flapping around when the vehicle is in the paint oven, where there's air circulating.

Once the vehicle is bagged and taped, it goes into the paint booth so that the second color is sprayed, then into the paint oven. Sheridan points out that there is another concern, which is that any overspray that goes onto the bag adheres to the bag, because it would be unfortunate to have the job done and then bits of paint flaking down onto the freshly painted surface while the vehicle is in the oven. The 3M masking materials are formulated to keep that from happening.

From a cost standpoint, the benefit is fairly straightforward: the amount of paint used is directly related to the area where the second color is on the vehicle. It is not a complete second painting. Given that the price of one of the bags is on the order of from $4 to $6, the savings can be realized in short order.

In addition to which, Sheridan notes that best efforts notwithstanding, things tend to go wrong in vehicle builds that sometimes necessitate repairs on the paint. Which can mean that the whole vehicle is brought back through paint. Or, as a more economical alternative, to use the bags or sheets to mask off all but the area where the repair is required.

It may be that the paint problem is caught at the end of the line. Some exterior parts are too heat-sensitive to withstand the paint oven, so the vehicle is not going to be stripped of parts and sent back through the oven. Rather, there will be paint touch-up like those used in repair shops that many car owners are, unfortunately, familiar with. The paint is cured through the use of infrared (IR) lamps. To deal with the issue of those parts that can't take the heat, 3M has developed a polyester film with an aluminum coating that can be used to mask the heat-sensitive parts (e.g., fascias, side moldings, bumpers, tires, plastic wheels). The material will reject up to 70% of the heat load, up to 340oF, for one hour from IR lamps.

Sheridan admits that 3M isn't the only company that offers materials like these. But he maintains that it is the only company that offers a complete system (tapes, bags, films, etc.)