Main reason:

1.The underly paint is not match with surface which make the underly paint and the paint not easy to produce reaction. 

2.Without the underly paint drying, paint it again on the surface. 

3.Paint is too thick.

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

Precaution:

1.Test it firstly to make sure it correct color.

2.Use the sandpaper to rough the metal surface and spray the antirust primer before spraying.

3.Remove aging paint.

Correction method:

Remove the old paint or use the Aodalin paint remover to get the bare metal surface and then spray antirust primer with spray paint.