How to smooth rough areas with filler | Basic body work part 2

Solid Foundation

In the first part of our Basic Body Work series, we fixed some rusty and damaged body sections by welding in new sheet metal. That’s the most important part of the whole process, as a solid foundation ensures that any labor and money spent from that point forward aren’t wasted. Our base will be strong enough to not crack or deform under load and will be corrosion-proofed so that rust won’t bubble back to the surface. 

Polyester filler is used to smooth out the bodywork so that the repairs look sharp once it’s painted. While lead was once used in the manufacturing and repair process, it has been set aside for environmental and health concerns. Modern polyester fillers have improved to the point where their results are superior to lead’s—they’re now used by all major auto makers on the factory floor. 

Do yourself and your project a favor and head to your local body shop supply company for some advice. Ask them to name some easy-to-use polyester fillers that sand and age well. 

The counterman at our local shop, Higgs Auto Paint and Body Supplies of Holly Hill, Fla., recommended Half Time. True to his recommendation, Half Time cures quickly and sands easily, making the work go smoothly in more ways than one.

Step 1: Get the Mix Right

Photography Credit: Per Schroeder

Polyester fillers come with a small tube of methyl ethyl ketone peroxide hardener. Only a small tube is needed because the typical filler-to-hardener ratio is about 40:1. 

Our Half Time filler has a recommended ratio of 50:1. We used a small postal scale to get a better feel for this ratio. 

So, why is the ratio important? The hardener creates heat through a chemical reaction with the styrene in the filler. There must be just the right amount of heat present to help cure the mix.

Since heat is such an important part of the process, the temperature of the working environment must also be considered. Most brands of fillers should be applied when the ambient temperature is between 60 and 80 degrees Fahrenheit; that’s the range that the chemists used when developing the hardener and filler mix. If you work with the filler when the air is too cold or too warm, the product will harden inconsistently and its strength will be compromised.

Step 2: Sling Your Mud

Photography Credit: Per Schroeder

Fillers should be applied over solid, bare metal or over sealing primers that have been roughed up a bit with sandpaper. The adhesion between the filler and the car body is mechanical, not chemical, so the surface has to be a little rugged for proper adherence.

When first starting out with fillers, the tendency is to mix up a much larger amount than you need, most of which will cure long before you get around to applying it. Start small. 

First, squirt out about an ounce of filler—a spot roughly an inch in diameter—onto a clean mixing board. Then dab a little bit of hardener into the middle of the spot. Mix it up with the filler spatula—a rectangular piece of plastic that is used as a trowel—and apply the filler to the car. 

Try to keep the filler to no more than 3/8 of an inch thick. If you find yourself needing more than that, then your metal work isn’t done. Break out the hammer and dolly and work on that section a little more.

Step 3: Shape and Sand

Photography Credit: Per Schroeder

When the filler cures—Half Time is ready in less than 10 minutes—start shaping it with a hand board or cheese grater-like rasp file. 

You should use a sanding block on flat surfaces to prevent waves in the finish. Larger blocks are required for the larger panels that need to be perfectly pin-straight. You can hand-hold sandpaper for curved surfaces, but it’s important to keep the contours correctly shaped when working this way.

After you’ve roughed out the shape of the repair, go to an 80 grit paper. We’ve had good luck with handheld electric sanders and air-powered dual-action sanders. 

If you’re noticing that the shape isn’t quite right after sanding with 80 grit, mix up some more filler and continue the application process. It doesn’t make sense to keep sanding the surface if there are large imperfections that still need to be filled. 

Once you get to the point where the surface is smooth and correctly shaped with the rough sandpaper, step up to 120 grit paper. After the 120 grit, go even finer with 220.

The goal is to create a finish that is smooth to the touch. Here’s a trick: Run both the fronts and backs of your fingers over the repair area as the backs can be more sensitive than the fronts. Run your hand back and forth over the surface to pick out any areas that need attention and keep at the sanding until the entire repair feels and looks perfect.

Step 4: Prep for Paint

Photography Credit: Per Schroeder

After all the filler has been applied and roughed into shape, you can lay down a coat of high-build primer. “High-build” refers to the fact that the filler has a larger concentration of solids. Once its volatile content evaporates, a thick but sandable coating remains. This primer can then be block sanded down with 220 and 400 grit sandpaper to eliminate any remaining imperfections before the top coat is applied. 

To make sure that what you think is straight really is straight, you can apply a thin guide coat of a contrasting primer. Typically, a darker primer is sprayed over lighter shades of build primer.

As the guide coat of primer is sanded down, the high spots will appear as large sections of the lighter primer base and the low spots will remain dark. (The contrast is very subtle and therefore hard to photograph.) You might have to use spot or glazing putty to fill in these small imperfections. As with the rest of the steps, repeat until perfect.