Ratch has covered a lot of this, so I'll confine myself to the Vallejo questions. Let me start by saying that I don't really like them, so don't use them much. That said, I don't find that they need thinned when I do use them, but do find that some ranges have some colours that take much longer than others to dry, so it would be helpful if you said which colours you have issues with. OK? Now, on decal fixatives. Don't use a setting agent unless you have to or it's recommended in the instructions (for example on a Ford or Junkers trimotor with a corrugated skin, or carbon fibre effect decals on anything). If you do use a setting agent, apply it whilst the decal is still wet, then LEAVE IT ALONE until it's dry.