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5 myths about polyester putties

Myth #1. Polyester putty with aluminum particles is designed  only for aluminum body parts.

This is a very common, for reasons unknown to me, misunderstanding about this very useful material. To understand this better, it should be clarified that it’s a resin, which affects adhesion, not a type of filler used in polyester putty.

In our case aluminum particles are used for the following 3 reasons:

1. Aluminum is stronger filler than talc in ordinary fillers or glass balls in light ones. This is why putty with alu particles should be used on body parts subject to mechanical stress.

2. Aluminum, unlike talc, doesn't absorb solvents. Therefore putty with aluminium shrinks no more than 1%, while talc based fillers may shrink up to 3%. Hence, polyester putty with alu may be used when a thick layer of filling is needed.

3. Due to its rich aluminum content, filler with aluminum can be used on car body parts where high temperatures are probable, like in engine compartment or on a bonnet. Aluminum particles help the heat to spread out without concentrating in one particular place, moreover it can be used as an intermediate layer in combination with other filling putty. Thus, the high  temperature will not cause any damage to the repaired part. In fact, putty with aluminum can stand temperatures of above 90° C without losing its properties, whereas ordinary product only up to 80° C.

3 putties

Myth #2. Resin on top of the putty means low quality or an expired product.

I have extensively discussed this subject with many colleagues of mine, and there is no justified reason to believe that a sign of resin on the top of the putty when it is opened, means any particular performance problem. What is very important though, is to mix the resin in the putty thoroughly, and under no circumstances to spill it out. A rule of thumb is that too dry consistency of polyester filler will be difficult to work with and over-usage of hardener may occur to make it more workable. While, too much resin separation makes it more difficult to mix and apply on vertical surfaces.


Myth #3. The most important characteristic of quality polyester filler is ease of sanding.

This is probably one of the most common myths about putty. No doubt, it is very important to have a product, which is easy to shape. Especially in growing markets, where sanding machines are not widely used, and all blocking jobs are done manually, ease of sanding is very helpful. However, the major purpose of polyester filler is to "fill" the surface and mask the damage. If polyester filler shrinks, it will reveal the repaired area with a contour. From my point of view, good polyester putty should show minimum shrinkage of less than 3% at first place. I would also put other important characteristics first, for instance, adhesion properties, absence of pinholes and ease of application.

Myth #4. The more hardener you use, the faster is the drying time.

All the manufacturers of polyester filler recommend using only 2-3% of hardener. Nevertheless, many car sprayers and panel beaters believe that more hardener will significantly improve the drying times. The bitter truth is that  the filler's molecules will bind only with a certain molecules of the hardener, and not more. In addition, excessive hardener will react with consecutive coatings, causing discoloration and adhesion problems.

  Too much hardener

Myth #5. Light putty replaces all the other types of filling putties.

It is true that nowadays more and more body shops use light putties as a primary filling product. These materials are easy to sand, they have low shrinkage ratio, plus on a thin metal used in contemporary cars, lightweight putty causes less vibration. Nevertheless, light putties (with glass or plastic balls as fillers) are less rigid than common putties with talc, aluminum or fiberglass fillers. Hence, we can not use them on rusted areas or where mechanical stress is significant.



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