Personally, I like AK Interactive or AMMO MiG Jimenez products. They are both ideal for filling gaps or to refine some surfaces. They can both be easily sanded when fully dry and can even be “wetted” to make them smoother. They both come in dispenser tubes and a small amount goes indeed a long way. They both can be cleaned with water.
And here they are:
AK Interactive White Putty
AK Interactive Grey Putty
AK Interactive Green Putty
And AMMO MiG Arming Putty:
AMMO MiG also sells special Putty Bottles with a tiny brush applicator for those refined and difficult zones to reach on your models.
They come in three specific density gradations, thin, medium and thick, depending on what you have to patch up or how you want to finish your model.
And here they are:
Did I forget to mention? They are all acrylic based and are harmless to your health.