Body Filler and Glazing Putty for Beginners: Mixing, Applying, and Shaping
Body Filler and Glazing Putty for Beginners: Mixing, Applying, and Shaping
Introduction
Almost every dent repair ends with filler. Many people call all of it "Bondo," but a body shop uses three different products, and each has its own job. Use the wrong one, or mix it wrong, and the repair can crack, sink, or show tiny holes through the paint.
This is a beginner’s lesson. In Part 3 of our Kia Forte series, you saw this work on a real car; here we explain the basics behind it, from manufacturers and trade sources. At the end, in From the Shop Floor, KJ shares notes from about 20 years in our shop.
Key Takeaways
- Fiberglass filler gives strength, body filler gives shape, glazing putty gives the finish, in that order.
- Mix about 2% hardener by weight (≈ 50:1), same brand. Never extra on a cold day.
- Thin layers beat one thick layer. No more than 1/4 inch of filler, 1/8 inch of putty (3M).
- Shave high spots with a cheese grater in the green stage, before the filler is fully hard.
- Warm cold filler gently with a moving heat gun.
- Heavy filler on doors can crack. The owner should know the risk first.
Body filler and putty at a glance.
Learning Objectives
Fix the metal first. A dent is pushed or pulled as close to its shape as possible before any filler goes on. Filler fills what is left; it does not replace metalwork.
Safety: sanding filler makes fine dust. Wear a respirator, eye protection, and gloves.
One rule: every filler, putty, and hardener has a technical data sheet (TDS). The numbers here come from manufacturers; your product’s TDS always comes first.
1. Fiberglass Filler, Body Filler, and Glazing Putty
All three are polyester products that harden when you mix in a hardener. The difference is what is inside them and how thick and smooth they are.
Figure 1. Three products, three jobs.
I-CAR describes fiberglass-reinforced filler as the choice for added strength, especially over weld seams and on SMC or fiberglass, and says it must be finished with body filler or putty before primer. Evercoat calls putty "basically a thin body filler designed for finish work." You will also hear about metal-reinforced fillers (with aluminum), which are stronger and more water resistant.
Why can’t fiberglass filler be the last layer before primer?
2. Mixing Filler and Hardener
Filler cures by a chemical reaction with a cream hardener. Too little, and it stays soft and sticky. Too much, and it can cure badly and cause problems later.
Figure 2. Mixing ratio, and how temperature changes it.
Most polyester fillers use about 2% hardener by weight (about 50:1). Evercoat’s rule of thumb is a ribbon of hardener edge to edge across a 4-inch puddle; 3M’s short-strand TDS allows 1.5 to 2.75%. Use the same brand of hardener. Mix on a clean, non-porous board until the color is even: streaks mean some filler has no hardener, and that spot stays soft.
Heat makes filler set faster and cold makes it slower; Evercoat recommends applying it between 60 and 90°F. On a cold day, do not add extra hardener (3M warns against it). Give it time, or warm it gently (Section 5).
On a hot summer day, filler can be too stiff to use in about 3 minutes. See KJ’s Note 1.
On a cold day, why shouldn’t you add more hardener?
3. Applying Filler, and How Thick Is Too Thick
Figure 3. How thick is too thick?
Caution: Thick Filler Can Crack
A panel repaired with filler or putty can crack, and the thicker the filler, the higher the risk. Panels that take repeated shocks are the biggest concern: the driver’s and front passenger’s doors open and close many times a day, and every slam sends a shock through the filler. That is why the metal is straightened first, and why the owner should know the risk before the repair starts.
Figure 4. Why front doors crack first.
Past about 1/2 inch, cracking becomes much more likely, and the owner must agree before the work. See KJ’s Note 5.
Why are driver and passenger doors more likely to crack when they have a lot of filler?
4. Shaping With a Cheese Grater Blade
A cheese grater blade is a long, half-round file with many small cutting holes. A 10-inch blade fits a standard file holder and shaves filler, fiberglass, and plastic quickly. Filler goes through three stages after mixing: wet, green (firm but a little rubbery), and fully hard. The grater is for the green stage.
Figure 5. Use the cheese grater in the green stage.
Fully cured filler and fiberglass get extremely hard. See KJ’s Note 2.
Why is it better to use the cheese grater before the filler is fully hard?
5. When Filler Won’t Dry: Using a Heat Gun
In cold weather, filler and putty can take a long time to cure, and gentle heat helps. Evercoat advises not to exceed 120°F (50°C) for more than 15 minutes. More heat is not better.
Figure 6. Warming cold filler with a heat gun.
See KJ’s Note 3 for how we use the heat gun in winter.
Name two rules for using a heat gun on filler.
6. Filling Pinholes and Fine Scratches Before Primer
Even good work can leave micro-pinholes and fine scratches that are hard to see before primer. If they are not filled, they show through the paint. A micro-pinhole eliminator, such as Evercoat 440 Express (a one-part polyester), is made for this. Evercoat says that, used properly, it removes up to 90% of the rework caused by micro-pinholes.
Figure 7. Where each product goes, from metal to primer.
We also use it on fine scratches from bodywork. See KJ’s Note 4.
What does a micro-pinhole eliminator fill, and when is it used?
Common Misconceptions
Try It Yourself: Two Batches, Two Temperatures
Quick Review
Frequently Asked Questions (FAQ)
Glossary
Review Questions (with Answers)
1Why can’t fiberglass filler be the last layer?SHOW ANSWER ▾
2Why no extra hardener on a cold day?SHOW ANSWER ▾
3Maximum filler thickness?SHOW ANSWER ▾
4Why are doors more likely to crack with heavy filler?SHOW ANSWER ▾
5Why use the cheese grater in the green stage?SHOW ANSWER ▾
6Two heat gun rules?SHOW ANSWER ▾
7What does 440 Express fill, and when?SHOW ANSWER ▾
Shop Tips
- ✓Mix small batches in summer. You get about 3 minutes before it stiffens.
- ✓Keep the cheese grater close. The best time to use it comes and goes fast.
- ✓Thin and tight beats thick and fast.
- ✓Wipe a pinhole eliminator before primer, even when you cannot see any pinholes.
- ✓Warn the owner about heavy filler on doors, and get their OK before you start.
From the Shop Floor: KJ’s Notes
Sources and Related Reading
Previous and Next Lessons
About the Author
This lesson is written by the team at Ace Auto Body Inc. in Hackensack, New Jersey, an auto body repair and paint shop, with about 20 years of experience behind KJ Lee's work. We share real repairs from our shop, with our own diagrams, so car owners can understand what a proper repair involves. Read more on the About page.
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