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Plastic-Filler Fundamentals

Shaping plastic filler is considered one of the fundamental skills needed by entry-level collision technicians. Some entry-level technicians grasp this skill quickly, while others struggle. Why is this? Part of the difficulty stems from the inability to see the entire forest for the trees. You see the dent or rust-out repair, so you focus on that one little spot. And you concentrate so hard on fixing that spot you forget that when you’re done, the repair must match the shape of the panel.

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Time is money in this business, so the quicker you master this skill, the better. I can remember my first attempts at filling small dents. It took forever, and I wasted five times more filler than was actually required. I always seemed to sand away too much filler, or I just couldn’t quite get the repair area "straight" again. Eventually, this particular skill became second nature to me. If you’re also having trouble mastering this skill, have you ever asked yourself what shaping plastic filler really is? What does it involve? How would you explain it to someone? Sometimes clarifying what you want to do so you can explain it to someone reinforces your own learning. Let’s see if it works in this case.

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Getting In Shape
Straightening a dent or a rust repair with plastic filler doesn’t really mean "straight." How many panels on a modern vehicle are actually straight or flat? Not very many. If you do encounter a flat surface, it isn’t going to be very large. So "straight" doesn’t really mean straight. A better definition would be to restore a panel to its original shape and contour, which ultimately means a mix of flat, convex and concave surfaces.

Good plastic-filler shaping skills start with good metal-working skills. If you’re doing a lot of hammer and dolly work (metal work), it’s absolutely vital that you check for high spots prior to applying that first coat of filler. The human eye is an excellent tool for visually enjoying a beautiful sunset or watching television, but it’s not the best way to check a panel for high and low spots. The problem with high spots is that if you apply filler and then sand it down only then to discover high spots, you have to start over.

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You also need to make sure you apply a sufficient amount of filler so you bridge over the repair. If you don’t put enough filler in the low area, you’ll never get the panel restored to its original shape and contour. Don’t waste filler, but make sure you use enough. And don’t try to fill a repair with just one coat or layer of filler. Apply one coat of filler, sand it down and apply another coat. One heavy, thick coat usually means most of the filler ends up on the floor and in the waste bucket.

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Most of the techs I’ve observed use some type of slow-speed sander with 36-grit paper to remove paint and prepare the metal for plastic filler. This air tool and sandpaper combination will leave a particular pattern in the steel panel. If, after removing the paint, you take a flat sanding tool with 80-grit paper and sand across and then up and down, you’ll immediately locate the high and low spots. Remember, just a few strokes. If the low spots are more than 1/4 inch, you should bring up these areas to within 1/8 inch.

The second part of the initial work is to make sure that if the repair area is stretched, you shrink the metal so the panel is tight. There’s absolutely no way a modern panel can be straightened with plastic if every time you start to sand, the panel moves in and out.

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Power vs. Palm
In the interest of productivity, use power tools as often as possible. These tools save valuable time but, keep in mind, smaller areas may actually be completed faster by hand sanding because there’s less risk you’ll remove too much filler.

If you’re going to use power tools, ask yourself what kind of air tools are available to make the work go fast. You have DA sanders, both 6-inch and 8-inch versions, and you have straight-line sanders. The secret to working with these tools is to keep the sanding paper flat on the filler. The other secret is to use only the amount of pressure needed. When you first begin sanding, use more pressure. As the removal of plastic filler continues, you then decrease the pressure. Remember, more pressure results in a higher rate of plastic removal, while less pressure slows down the process.

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Directional movement of the air tools will ultimately determine success or failure when sanding plastic filler. The back-and-forth or up-and-down movement is good to get started and to sort of level off the filler. Once you get started, some sanding strokes at a 45-degree angle from the upper left corner to the lower right corner and more strokes at the same angle from the upper right corner to the lower left corner will help eliminate potential ridges. It’s good to always finish up with sanding strokes that follow body lines.

Sanding Secrets
Air tools are fast, and experienced techs can do wonders with them. But discipline yourself to always finish up with hand sanding. Hand sanding will put the "straight" in your work.

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A number of hand-sanding tools are available, all of which serve a purpose. Flat sanding blocks in several lengths are necessary. Large areas require a longer block than do smaller areas. Something to sand concave areas is also useful. For concave-surface sanding, you can either purchase a special sanding block, or some technicians keep several diameters of clean radiator hose in their toolboxes.

Another useful bit of information is that filler sands much faster than steel. This certainly isn’t a revelation, but sometimes inexperienced techs forget this. If you concentrate all your effort on sanding filler the way you would steel, you’ll remove too much and have to start again. Also, don’t forget about directional movement; keep your sanding tool moving, changing location and direction often. Several strokes here, then several strokes a couple of inches to the right. Change direction, a couple strokes here, then a couple strokes there — always moving with the contour of the panel.

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Another secret: New sandpaper is sharper and, therefore, will cut more aggressively and help keep the filler straight. If you’re working on several spots or one large area, rough sand each area while the sandpaper is sharp. This gets initial shape. Then go back over the area again after the sandpaper has dulled down a little.

Re-shaping a damaged panel is a lot like art work. If you can see the entire panel and move your sanding strokes in the direction of the body lines, your work will flow with the surrounding panels. This concept is useful when repairing two or more panels in sequence. What if you were repairing a quarter, door and front fender? It’s not enough to straighten one panel at a time. Each repair must line up with the adjacent panel.

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The Big Picture
These so-called secrets aren’t really secrets but rather basic skills requiring a certain touch or feel. And, since there aren’t any high-tech gizmos out there you can program to do this task for you, you’ll have to learn the skills yourself by practicing them. Practice and repetition equal skill mastery.

One way to learn is to watch others who are more skilled. When you watch another tech, ask yourself: Why did he start sanding where he did? What strokes is he using? What direction is he sanding in? What tools is he using?

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Many experienced techs make filler "straightening" look easy; that’s as it should be. Yet many of those same techs would probably have some trouble explaining what they do. That’s because this skill has become second nature to them.

Until you grasp the basic fundamentals, the skill part will allude you. Hopefully, articles like this will help shorten your learning curve.

Experienced techs may not speak much about their skills or be able to explain the steps required to shape plastic filler, but the quality of their work speaks volumes. Until sanding and shaping become second nature to you, too, watch, listen and learn.

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Writer Fred Kjeld, a 20-year veteran of the collision repair industry, is an instructor at the Hawkeye Institute of Technology and a contributing editor to BodyShop Business.

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