Putty is used to fill gaps, seams or filling in any imperfections. It can also be used for changing shapes or building up areas. There are several types of putty or fillers and it depends on what you are using it for. Some shrink and do not take well to some paints. It is finding one that best meets your needs.
Berny
Phormer Phantom Phixer 
On the bench
TF-102A Delta Dagger, 32nd FIS, 54-1370, 1/48 scale. Monogram Pro Modeler with C&H conversion.
Revell F-4E Phantom II 33rd TFW, 58th TFS, 69-260, 1/32 scale.
Tamiya F-4D Phantom II, 13th TFS, 66-8711, 1/32 scale. F-4 Phantom Group Build.