How to Avoid Shrinking the Body Filler

by Chris Stevenson
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Body filler, like the 3M brand Bondo, contains a paste filler of fiberglass polyester resin and talc, along with a small amount of styrene that acts as a solvent and vaporizes. Body filler also has a hardening agent that serves as a catalyst to cure the two ingredients into a durable mass. A chemical reaction between the paste and hardener causes the curing process. Although body filler is formulated for non-shrinkage, this is entirely dependent on the ambient temperature, mixing process, hardener to paste ratio and metal surface preparation. Shrinking, bubbling and collapsing can result from any misstep in the application process. A do-it-yourself repairman must follow strict application steps to avoid the most common shrinkage problems.

Step 1

Plan to apply body filler to a reasonably small area of repair, with a depth in the metal that does not exceed 1/2 inch, if possible. Confine the area to less than 6 inches in diameter. Overly thick concentrations of body filler are more prone to excessive shrinking than smaller areas. Mask off one localized area at a time, about 2 or 3 inches past the damaged area. The ambient outside temperature for perfect adhesion will be from 72 to 80 degrees Fahrenheit, or according to directions. Do not apply body filler when the temperature is under 64 degrees or over 95 degrees.

Step 2

Attach a 16- or 24-grit grinding disc to an orbital sander and grind the damaged area down to bare metal, overlapping the area by 1 to 2 inches. Use vertical and horizontal grinding strokes to achieve a crosshatch pattern. For hard to reach creases, sand with finger pressure by using a coarse 180-grit wet sandpaper. Blow the dust away with compressed air.

Step 3

Use a can of etching primer to spray a light coat over the bare metal surface. This will stop immediate oxidation, until you apply the body filler. Use a hairdryer or heat gun to heat the metal surface to a temperature that feels warm to the touch -- this will eliminate moisture contact on the metal if the humidity is high. Pour a small mound of body paste that measures approximately 3 inches in diameter and 1/2-inch thick onto a clean, glossy piece of cardboard.

Step 4

Squeeze a medium-thick line of hardener agent over the top of the paste pile that measures 3 inches long. The exact ratio of hardener to paste is 1 1/2 to 3 percent hardener to the paste volume. Use the kit spatula to thoroughly mix the hardener into the paste, using lapping strokes from all angles. Pick up and press the ingredients down, then lift up and swirl several times. You will need a solid paste hue, indicating that the hardener has mixed completely.

Step 5

Wave the hairdryer over the metal surface again, until the metal feels warm to the touch. Quickly load a small amount of body filler on the kit spatula and spread it over the damaged area. Apply a coating no more than 1/4-inch thick, or 3/8-inch for a maximum thickness. Press the body filler down firmly from the top side, then the bottom and from both sides, removing all the air. Drag the spatula lightly over the top surface to form a smooth surface covering. Allow the body filler to dry, according to directions.

Step 6

Mix a new batch of body filler as you did for the first application, but use a new piece of cardboard for a pallet and a cleaned spatula. Most body filler will dry or set in about 1 hour, allowing a second application. Reheat the metal with the hairdryer until warm. Apply a second coat of body filler over the damaged area, raising the surface of the filler over the top surface of the metal. Apply firm pressure from all directions. Use the spatula to feather the ends of the filler edges outward from the middle of the damaged area until all bare metal is covered. Allow to dry.

Step 7

Apply a third coat of body filler to the damaged area if you have not covered the depth of the indentation. Allow the body filler to cure completely, according to the directions or for at least 24 hours. Move the metal piece, component or vehicle indoors, so it will not be affected by outside moisture or a temperature drop. If it must be left outdoors, loosely cover the piece or damaged area with plastic and masking tape.

Step 8

Use the orbital sander, with a 36-grit disc, to sand the top layer of the body filler when it has cured. Use very light strokes. Take the surface of the filler material down until it is almost even with the surrounding metal profile. Use a sanding block equipped with 180-grit wet sandpaper to further sand the body filler area. Switch to progressively finer grain wet sandpapers, to sand down the body filler until the filler material is smooth and even with the metal profile. For instance, start with 400-grit, then 600-grit and finish with 800-grit.

Step 9

Spray the body filler area with a sealer-primer and allow it to dry. You are now prepped for your original paint, plus a clear coat if desired. Wash the project piece or vehicle 2 to 3 weeks after painting, and continue washing as your regular routine dictates.

Step 10

Store the vehicle in a garage or covered shelter if you have performed body work with body filler. If the vehicle is to be left outside, cover it with a tarp or form-fitting car cover. Body filler, due to its nature, expands and contracts in response to shifts in temperature and humidity. Body filler can shrink with exposure to severe elements, like rain, sleet and snow. High humidity allows moisture to congregate in minute cracks, between layers of paint and inside body filler adhesion points to the metal.

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