Main reason:

1.The substrate has oil, water, dust and stains.

2.The base metal surface is too smooth or without polished. 

3.The underly paint has been aging, chalking and brittle rupture.

Precaution:

1.Ensure the surface is completely clean.

2.After spraying, use the sandpaper to polish the metal surface and spray antirust primer. 

3.Remove the old paint and then paint.

Correction method:

Remove the peeling part and the surrounding weak attachment part and then use the P800 sandpaper to polish and spray again.