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.