When 4S shop technicians perform repairs, applying auto body filler (bondo) is a critical step. The quality of this application directly affects the final paint job's outcome, and technique is everything. Let's break down the professional process:
1. Mixing the Body Filler
Use two spreaders for the job
Scoop filler from the can onto a mixing board using one spreader
Add hardener at 100:2 to 100:3 ratio (filler to hardener)
Use the spreader tip to evenly distribute hardener across the filler surface
Lift the spreader edge, slide it under the mixture, and fold toward the board's left side
When about 1/3 is lifted, pivot on the spreader's right edge and flip it
Keep the spreader parallel to the board while pressing down
Repeat until thoroughly mixed (work quickly - you have just minutes before it sets)
2. Application Technique (Demonstrated on Rear Fender)
Grip the spreader between thumb and middle finger, with index finger on top
Load filler onto the spreader from your mixing board
Apply in downward strokes following the fender's curvature
Work systematically, returning excess to the board
Three-layer application method:
First Pass:
Fills major depressions
Press firmly for maximum adhesion
Focus on leveling, not smoothness
Second Pass:
Addresses remaining depressions (thinner layer)
Use long strokes to minimize seams
Avoid trapping air (prevents pinholes/cracking)
Third Pass:
Fills minor imperfections/sanding marks
Apply pressure for smooth, pore-free finish
3. Sanding Process
Drying: Air dry or use infrared lamp
Wet Sanding (Recommended for Partial Repairs):
Rough sanding (120-grit):
Achieve 80-90% levelness
Continuously check flatness by hand
Contour sanding (320-grit):
Follow fender curvature with slight downward angle
Remove previous sanding marks
Maintain original panel contours
Fine sanding (600-grit):
Complete surface coverage
Eliminate all visible sanding marks
Full panel prep (1500-grit):
Entire fender (blend into 1/3 of C-pillar)
Degloss existing paint
4. Final Touch-ups
Inspect for pinholes/sanding marks
Apply glazing putty (red filler):
Press small amounts directly into imperfections
Multiple thin layers for deep holes
Final sand (600-grit with soft block):
Remove excess putty
Leave only filled cavities
When 4S shop technicians perform repairs, applying auto body filler (bondo) is a critical step. The quality of this application directly affects the final paint job's outcome, and technique is everything. Let's break down the professional process:
1. Mixing the Body Filler
Use two spreaders for the job
Scoop filler from the can onto a mixing board using one spreader
Add hardener at 100:2 to 100:3 ratio (filler to hardener)
Use the spreader tip to evenly distribute hardener across the filler surface
Lift the spreader edge, slide it under the mixture, and fold toward the board's left side
When about 1/3 is lifted, pivot on the spreader's right edge and flip it
Keep the spreader parallel to the board while pressing down
Repeat until thoroughly mixed (work quickly - you have just minutes before it sets)
2. Application Technique (Demonstrated on Rear Fender)
Grip the spreader between thumb and middle finger, with index finger on top
Load filler onto the spreader from your mixing board
Apply in downward strokes following the fender's curvature
Work systematically, returning excess to the board
Three-layer application method:
First Pass:
Fills major depressions
Press firmly for maximum adhesion
Focus on leveling, not smoothness
Second Pass:
Addresses remaining depressions (thinner layer)
Use long strokes to minimize seams
Avoid trapping air (prevents pinholes/cracking)
Third Pass:
Fills minor imperfections/sanding marks
Apply pressure for smooth, pore-free finish
3. Sanding Process
Drying: Air dry or use infrared lamp
Wet Sanding (Recommended for Partial Repairs):
Rough sanding (120-grit):
Achieve 80-90% levelness
Continuously check flatness by hand
Contour sanding (320-grit):
Follow fender curvature with slight downward angle
Remove previous sanding marks
Maintain original panel contours
Fine sanding (600-grit):
Complete surface coverage
Eliminate all visible sanding marks
Full panel prep (1500-grit):
Entire fender (blend into 1/3 of C-pillar)
Degloss existing paint
4. Final Touch-ups
Inspect for pinholes/sanding marks
Apply glazing putty (red filler):
Press small amounts directly into imperfections
Multiple thin layers for deep holes
Final sand (600-grit with soft block):
Remove excess putty
Leave only filled cavities
