Micro Set is what you use first to help the decal adhere to your model. Micro Sol is what you use to help the decal conform to the irregularities of your model's surface. In some cases, you can get away with not using Micro Set, but it's still a good idea for best decal adhesion. This is especially true if you'll be moving the decal around a lot or if you realized you applied it to the wrong spot and have to move it to another part of your model.