Several types of Bondo products are available --- each for a particular purpose. Standard Bondo filler in regular and light weight are developed for the sole purpose of filling small dents, such as parking lot dings and similar damage. They can be applied to larger areas, although they have no structural integrity. They adhere exceptionally well and are easy to work with. Bondo glass is a different color, it is green and contains fiberglass fibers. It is extremely hard when cured, unlike regular body filler. Bondo Glass works well for filling rust holes. It has fiberglass matting fibers that strengthen the repair. Bondo fiberglass is the strongest structurally and can tie metal parts together and span larger openings or can be formed to replace rusted parts. It all depends on your particular abilities when it comes to forming parts. It is usually stronger than the original part and will not rust.
Sand the area down to the metal and overlap the dent by 1 inch. Spray the area with Ospho. Open the can of light body filler and place enough body filler on a piece of cardboard. You must mix the bondo with the included activator for it to to harden. This is the case with all Bondo fillers and fiberglass. The thing to keep in mind is that the body filler, once activated, will harden to the point of no longer being pliable within two minutes. Have the body filler spreader and everything else out of the way so you can apply it quickly.
Apply two drops of activator to the body filler and quickly mix the body filler and activator until it changes color. Waste no time; spread the filler in the dent and taper the filler out for an inch around the area. Smooth the filler out, leaving it just a little higher than the dent. Body filler shrinks as it dries.
Allow an hour for the filler to cure and shrink before sanding it. Moving in several directions with the block sander, sand the area level. Sand the area with 1000-grit, wet-and-dry sandpaper. Wet the sandaper constantly with water. This will eliminate scratching.
Spray a very light coat of black paint over the area. Rub the block sander over the area lightly, looking for high or low spots. The paint will remain in any low spots.
Fill the low areas with more body filler and repeat the process. Sand the area again, using 2000-grit, wet-and-dry sandpaper to finish the repair. Primer the area with primer filler. Wet-sand the primer after an hour with 2000-grit, wet-and-dry, and it is ready for paint.
Sand the area down to the bare metal. Spray the area with Ospho. Drill several holes around the dent, but lower than the level surface. This gives added adhesion for larger areas. Some of the body filler will penetrate the holes and harden, helping to keep the repair solid.
Place enough Bondo glass on a piece of cardboard and mix in a proportionate amount of activator. Here is a tip for adding activator: for every 6 ounces of filler use a 1/2-inch bead of activator. Remember, the more activator, the faster it sets. To much activator and it sets before you even finish mixing it. Use just enough activator to change the color of the filler, and apply it immediately. Work quickly and mix the two together. With the plastic applicator, push the filler into the holes with a little pressure so it passes through the holes. Fill the rest of the dent level with the surface. Allow it to set until it is not sticky to the touch. Depending on the temperature and humidity, this varies between 20 minutes to 2 hours.
Sand the areas level with the block sander and some rougher 300-grit sandpaper. Place enough light body filler on the piece of cardboard and add the activator.
Spread the body filler over the Bondo glass to fill the holes and produce a smoother and easier surface to work with. Follow the directions in the first section of this article for the finishing touches.
Cut the rusted section out with metal shears. Make every effort to keep the edges straight and unwrinkled. Grind an area 2 inches around the area to be repaired. Do so also on the backside of the metal. Make sure the area is ground clean to the metal.
Cut some fiberglass matting material so that it overlaps the hole by 2 inches on both sides. Spray some trim adhesive around the backside of the hole. Apply the fiberglass matting immediately to the backside of the hole. Make the matting taut with no wrinkles. Allow the adhesive to dry for a few minutes.
Put on gloves. Pour some fiberglass resin into the bowl provided. Place a rag under the hole to catch any spills. Get the spreader ready because the fiberglass resin is going to set very quickly once you mix it.
Mix in the appropriate amount of activator as per instructions on the can and mix well for 5 seconds. Hold the spreader under but touching the matting and pour a little resin at a time on the spreader and spread the resin over the fiberglass matting. Keep moving quickly and saturate all the matting completely. It will turn translucent as it is saturated.
Allow a few minutes for it to begin to set, and add one more layer of fiberglass matting. Since the resin has not completely set, the matting will stick to the installed matting. Mix more resin and saturate the second layer of fiberglass matting. Allow it to dry and set until it is no longer tacky. This is now a very strong backing repair to which body filler can be added to finish the repair. Finish the repair by following the instructions in the first section of this article.
