Today's vehicles are made with thinner metal panels and more complex body shapes than in the past, and that affects the damage repair process. Sheet metal problems that a mobile installer might face include things such as door dings, small holes, dents, hail damage, and creases and depressions that are 1/8-in. deep or less. Such damage generally can be repaired with body filler and/or putty. Fillers can be used to effectively fill body panel gaps or depressions, and where necessary, putty completes the top layer and provides a smooth, pin-hole and scratch-free finish, ready for painting.
There are three categories of body fillers for body repair: standard, mid-range and premium products, and these are two-part materials consisting of a polyester resin and a cream hardener. Individual fillers are formulated by the manufacturer to be compatible with various body materials, including steel, galvanized steel, aluminum and SMC.
In some cases, filler staining can be a problem. Staining is caused by a chemical reaction between the resin or hardener and automotive paint. Such discoloration can be minimized by selecting good quality filler, mixing it carefully according to instructions, and avoiding filler work when the humidity is high.
Filler Options
Minor depressions, dents and creases can be repaired using standard grade body filler. This category of filler is lightweight, spreads smoothly, typically cures to a tack-free state in 20 to 25 minutes, and is easy to sand. A standard filler is not appropriate for galvanized steel or SMC, or for use in applications where staining is likely to occur.
Medium grade fillers sand more readily than standard fillers, have better stain-resistance, and generally adhere well to all the various body substrates. This filler category includes fiberglass-reinforced fillers that are intended for repair of small holes, cracks and tears in fiberglass as well as small holes in metal.
Sprayable polyester fillers can be included in the medium grade filler category. This type of filler is used for repairing minor scratches and imperfections across a broad vehicle surface and is applied with a primer gun. Sprayable fillers help eliminate the staining that can occur on basecoat/clearcoat systems.
Finally, premium grade fillers are made with a premium resin that offers excellent stain resistance, are the easiest to sand, and provide the best adhesion of all filler materials to underlying surfaces.
Using Fillers
You need to mix a filler carefully to achieve good results, taking care to use the recommended amount of hardener and avoiding trapped air during handling. Begin by spreading the two components on a clean board, folding rather than whipping the material together. Then apply filler by spreading it on the body surface in thin layers and pushing it into grooves for a good mechanical bond. Press out any air bubbles and overbuild the repair area slightly. This provides sufficient base for sanding to a smooth, flat finish. Begin sanding the cured filler with an 80 grade paper, moving to 180 and then 240 grade papers in sequence to achieve a paintable surface.
Finishing the Job with Putty
It may be necessary to complete a filler repair job with a thin coat of flowable putty to eliminate tiny pinholes and achieve a completely smooth surface. Today's body repair putties are formulated for easy sanding and this allows the user to achieve cleaner results with less effort than in the past. Two-part polyester putties have become the primary option for filling very small dents, creases and body imperfections and to achieve a very smooth finish over thicker layers of filler.
Like fillers, body putties are generally two part polyester formulations. A polyester base is mixed with a hardener and cures quickly, making it possible to sand the surface in just 15 to 30 minutes, depending on the amount of hardener used and air temperature. A good quality putty will adhere well and shrink minimally as it cures, and will sand easily to a thin, feathered edge.
Very minor imperfections such as pinholes and sand scratches can be filled with a spot putty or a glazing putty, both of which dry relatively quickly. Glazing putties are slightly slower drying than spot putties, which allows them to be spread over larger areas. Both spot and glazing putties should be applied over primed surfaces for best results.
As with a filler, sand the cured putty with a sequence of abrasives beginning with grade 180 followed by a grade 240 or 320 for a smooth finish. It is important to wait until putty or finishing putty is fully cured before sanding to avoid shrinkage and abrasive loading.
Users should choose the highest quality fillers, putties and abrasives available because such materials will be the easiest to work with, suited to more than one kind of task, and help avoid errors that require rework. Be sure to read and carefully follow instructions.
