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How to Check If Bondo Filler Is Smooth at a Curve

Auto body filler such as Bondo brand is a two-part resin. It is applied in a clay-like consistency that dries hard. It can be shaped with rasps, files, abrasive discs and sandpaper to replicate the original shape of auto bodies, prior to an accident. With flat or near-flat repair areas, it's easier to see imperfections in the repair. Radii, especially compound radii, require multiple techniques to ensure the proper smooth contour.

Things You'll Need

Flexible contour gauge
Sheet steel
Fine-tipped marker
Rotary Tool
File
Carpenter's chalk
Fine sandpaper
Flexible board sander
Build-up primer

Instructions

- 1
  Use a flexible contour gauge to determine the correct cross section of the curved area you're repairing, pressing the gauge against a fender or body area on the opposite or undamaged side of the car.
- 2
  Lay the contour gauge on a piece of sheet steel to be used as a contour scraper. The steel should be approximately 1/8th of an inch thick. With a fine-tipped permanent marker, transfer the contour onto the sheet steel.
- 3
  Fabricate the contour scraper with a rotary tool or abrasive wheel, or by drilling multiple holes, then filing the remainder of the material away with a hind file. Even if you use a power tool, it's good to use a hand file for filing right up to the edge of the gauge.
- 4
  Hold the contour gauge against the undamaged section of the car to check it for accuracy. Make minor adjustments to the gauge with a file, if need be.
- 5
  Use your contour scraper to remove most of the remaining material, after you've completed your Bondo buildup, removed the bulk of the excess material and sanded almost all the remaining excess away. Working perpendicular to the surface of the repair, drag the scraper along the axis of the curve, applying pressure as you go.
- 6
  Wipe marking material on the Bondo so you can see the high marks the way machinists use dye. Wiped on carpenters chalk will work.
- 7
  Make more passes until there are no high or low spots.
- 8
  Sand the curve with very fine sandpaper, between 400 and 1200 grit, using a flexible board sander -- a rigid but bendable plastic backing board specifically for auto body sanding. The sanding block won't allow your fingers to push through, deforming the curve, and fine sandpaper will not create a flat spot. You may use build-up primer, allow it to dry, then sand it back down to level the slightest imperfections.

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