Car Parts and Accessories

Of all the developments that have helped the ‘Do It Yourself’ motorist, undoubtedly the most outstanding has been the invention and subsequent improvement of chemical body fillers. The modern body filler is basically an inert powder which is mixed with liquid resin to form a paste. The consistency of the paste varies with the amount of liquid resin that is added. A different consistency of paste is used for different applications. A hardener is added to the paste causing it to set. Once set, it can be rubbed down with emery paper to any required shape.

Filling dents

The most common job that filler is used for is to fill in a dent. If the dent is very deep it should first be knocked out from the other side, but be careful because a dent is really a section of stretched metal. If you try to knock it out too much it can suddenly turn into a bulge, which has to be hit with a hammer to turn it into a dent again.

If the dent is no more than 4 inch deep, it is safe to use filler. If you use filler to fill in dents deeper than about 1 inch. there is a chance that, due to its inflexibility in deep sections, the filler will break away. This is caused by vibration or differences in expansion and the temperature change between the filler and the metal.

Using a medium grade of emery paper, the dent is first rubbed down to remove the paintwork from its surrounding area. If the dent covers a large area, you may be tempted to use a rough emery paper. This usually results in more work later on with finer paper to get the scratches out. Use a finer grade emery paper to smooth out the newly exposed metal surrounding the dent. Every so often, the emery paper will start to slide over the area being rubbed down, instead of cutting into it. This means that the paint and metal flakes have clogged up the paper. It can easily be unclogged by dipping it into a bucket of water.

Removing grease

To remove any traces of grease, rub the whole area over with a clean cloth dampened in white spirit. Household detergent can be used, but white spirit is more effective. To help the filler key in, scratch a criss-cross pattern into the dent (but not the surrounding metal) with the tang of a file or screwdriver.

Mixing fillet

The filler will then have to be mixed. The most complicated type of filler is the one found in bodywork repair kits which contain glass fibre.

It consists of three parts—filler powder, liquid resin, and hardener. The filler powder itself is rather a heavy powder and when mixed with the liquid resin, forms a thick paste. To this paste the hardener is added, which causes the paste to set solid in anything from 10 to 30 minutes depending on the amount of filler used and the ambient temperature.

The reason for having the filler in three separate parts is so that the liquid resing and hardener can be mixed separately and used to harden or ‘cure’ the glass fibre contained in these kits.

A simpler type of filler is known as ‘pre-mixed‘ and has the filler powder and liquid resin ready mixed in one container and the hardener in another.

A further refinement of these fillers is a type known as `elastic’. This is available only as pre-mixed. It does the same job as ordinary filler except that instead of setting absolutely hard, it retains a certain amount of flexibility. This makes it less likely to crack when used to repair areas prone to sudden shock or vibration, such as boot lids and wings.

As with everything a gain in one direction is a loss elsewhere and one of the unfortunate characteristics of the elastic type filler is that it is not so resistant to water as the normal type. This means that the metal on which it is bonded tends to rust earlier.

But let us suppose that we are using the three-part filler. Mix the resin and powder in such proportions that they form a fairly stiff paste, but not so stiff that when left for a few seconds the paste will not move to form a flat, even surface. When mixing powder and resin, it is most important that it be stirred slowly and thoroughly.

If a beating motion is used, pockets of dry powder will form in the paste and when the repair is rubbed down at the final stage, large pin holes will appear. This will mean that more filler will have to be used to fill the pin holes. Next the hardener is added and laid in, using a flat wooden spatula or knife. Stirring when adding the hardener can cause air bubbles, which will again result in pin holes.

If you have bought a pre-mix paste, all you have to do is to mix the hardener with the required amount of paste, but remember that once the hardener is added you will have about 5 minutes before the filler starts to set.

Applying the paste

When repairing a vertical surface do not try to fill in more than about inch deep at a time, or the repair will tend to sag.

If the surface is horizontal you can apply almost 1- inch at a time, but should you apply it any deeper, the heat generated during hardening could cause the surrounding metal to expand and result in poor adhesion.

The first layer of filler should be pushed firmly into the dent using a spreader or a piece of card. When buying a kit, a piece of plastic called a spreader is usually supplied. Wait until the first layer has set quite solidly before adding any further layers. A good test is to stick your thumb-nail hard against the first layer. If there is any sign of an indentation, other than a scratch mark, you will have to wait. Also make sure that it has not only set but that it is cool. Should you put further layers on to a previous layer which is still warm, it will cause it to set far too rapidly. Apply further layers of filler until the repair is proud of the surrounding metal

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