Paint Repair - Metal Surface Treatment
1. Metal Surface Treatment (Epoxy Primer Coating)
When bare metal surfaces are exposed to air after sanding, corrosion will form quickly. Apply epoxy primer to the surface in order to prevent corrosion and to improve topcoat adhesion:
-1.
Mix the main agent with its additives and dilute it with the correct reducer/thinner.
-2.
Apply one or two layers using a spray gun or brush.
-3.
Allow the epoxy primer to dry.

Drying time should be around about 60 minutes at 68 °F (20 °C). Force-drying will improve the paint quality. Check for the epoxy primer manufacturer’s product spec for the actual required drying time.


2. Selection of Body Filler/Putty

There are different kinds of body filler or putty:
Metal filler is good for rough filling when thickness is needed.
Light-weight panel filler sands very smooth.
Polyester putty which dries fast and sands very smooth, etc.

Each has its own characteristics, so choose a body filler or putty most suitable for the work being done.
In the following work process example, we will use intermediate body filler/putty. This type has characteristics of both panel and polyester fillers.

3. Preparing Intermediate Filler/Putty
-1.
Stir the putty until it is a smooth consistency.
-2.
Remove the required amount of filler/putty and place it on a mixing board. Then mix with the correct amount of hardener.
-3.
Knead and mix the filler and hardener with a spatula until the color becomes even. Be careful not to entrap air in the filler.

4. Application of Intermediate Filler/Putty
Apply intermediate filler/putty with a spreader into the recess of the damaged area.

NOTE: When the old paint layer is a lacquer type, be careful not to put the filler/putty directly on the old paint surface. If it happens, the filled area mark show through the new topcoat paint.
Basics on Body Filler/Putty Application

Do not apply filler/putty in a single thick layer. Instead, apply it in several thinner layer with sufficient drying time allowed between each.

NOTE: Be careful not to trap air in the filler/putty.
1.
Covering:
At the first application of the filler/putty, stand the spatula nearly straight up apply the filler/putty very thinly so that it penetrates into the hairline sanding scratches.
2.
Thickening:
Build up the filler/putty using a spreader held at a small angle.
3.
Flattening:
If the spreader leaves unevenness filler/putty surface, remove them by flattening the spreader’s angle.
NOTE: Surface unevenness is cause by the step-shape at end of spreader. Don’t push the spatula too hard, and completely smooth the filler/putty before it starts to dry up.
NOTE: If a large amount of putty is applied at one time, air may be entrapped in the putty, causing air pockets.
Use the proper spreader direction and angle depending on the filler/putty surface desired for paint repair.
Filler/Putty Application on Curved Surface
1.
Apply filler/putty to the damaged area (a) with a plastic spreader.
2.
Apply filler/putty to the damaged area (c) with a rubber spreader.
3.
Apply filler/putty along the line on the reverse curved surface of the damaged area (b) using a rubber spreader.
Putty on Stamping Line

Use masking tape for applying filler/putty on a straight-line area like the stamping line.
1.
Place the masking tape along the upper and lower stamping lines of the damaged area (b).
2.
Apply filler/putty to the damaged area (a) and surface (c).
3.
Remove the masking tape after putty dries.
4.
Place the masking tape along the stamping lines of damaged area (a) and (c) and apply putty on the damaged area (b).
NOTE: Disposing Filler/Putty
Body filler or putty generates heat via thermal reaction while hardening.
Place leftover filler/putty into a fireproof disposal can.
Wait until the filler/putty cools down before disposing in a regular trash can.
Do not dispose of filler/putty with flammable materials due to the risk of fire.

5. Drying Intermediate Filler/Putty
The drying time depends greatly upon ambient temperature. Filler/putty thickness also affects the drying time because a thermal reaction takes place while it is drying. Thicker filler/putty layers trap heat, increasing the internal temperature and accelerating the drying time.

NOTE:
Check the filler/putty manufacturer’s instructions for drying time.
Before starting forced drying, allow sufficient set-up time. Force dry at a temperature of approximately 122?140 °F (50?60 °C). Higher temperatures may degrade the hardener.
Avoid rapid heating or cooling as it may cause distortion of the steel body panel and cracking or peeling of the filler/putty.

6. Sanding of Intermediate Filler/Putty
1.
Sanding with Sander (Rough Sanding):
Use a double action sander or orbital sander with P80 to P120 sandpaper depending on the filler/putty condition and sanding surface area. Sand the filler/putty surface to a rough surface shape. Filler/putty must be dry-sanded.
NOTE: Use a dry sprayed guide coat to visually check the flatness of the filled surface.
2.
Manual Sanding (Rough Surfacing):
While feeling the rough sanded surface with your fingertips, bring the surface to the final finish using P120 to P180 sandpaper together with a file and a rubber block.
NOTE: Be careful not to sand the area excessively.
3.
Confirmation after Intermediate Filler/Putty Surfacing:
Check the surface for cavities or hairline scratches by feeling it with your fingertips. Then move on to the primer-surfacer process.
If a dent could not be filled with body filler or putty the first time, or there are low spots apply additional filler/putty again to correct them. Use polyester filler/putty in this case.
Use a surform tool when rough sanding wax type metal filler because it is difficult to sand.
Sanding becomes more difficult after the filler/putty hardens too much. It is the best to sand approximately 2 to 10 minutes from the start of hardening. Note that the edges of body filler/putty adhere slower than the center, so start sanding from the center in a radial direction.
NOTE: After a metal puttied surface is sanded, a number of hollow cavities remain on its surface. Because it is difficult to fill tiny air cavities with a primer-surfacer, use polyester filler/putty instead.

7. Cleaning and Degreasing

Blow air on the filled surface and remove any oily residue with solvent.

NOTE: Remove sanding residues from cavity holes.

8. Preparation of Polyester Filler/Putty

Mix the filler/putty with the required amount of hardener and knead it until its color becomes even.

9. Application of Polyester Filler/Putty
Apply polyester filler/putty.


10. Drying Polyester Filler/Putty

Dry polyester filler/putty.

11. Sanding and Finishing Polyester Filler/Putty
Use P180 to P240 sandpaper when manually sanding.
Use P120 to P180 sandpaper when using a disc sander.

NOTE: Be careful not to sand excessively.


12. Featheredge (Scuffing for basecoat)
Depending upon the situation, make the sanded old paint layer an even slope without steps.
For the old paint layer where primer-surfacer will be applied, scuff it with sandpaper for better adhesion to the new paint