Description of problem:
    During oven drying, the air withheld underneath the paint in the primer / fillers, the putty, the welding joint, etc… expands and bursts through the paint film.

Causes:         

• Bad application of the putty.  Putty is applied in thick layers and air allowed to be trapped in.
• Poor sanding

Preventive measures:

• Apply the putty with the proper application technique.

      Do not incline the knife at a small angle with respect to the surface as this allows the putty to roll under the knife.

• Sand well the edges.

Remedy:                         
    Remove the paint all the way to the bare metal, clean with Valox (based on phosphoric acid), prime and paint.