Two ingredients that never show up in the finished film the way a binder or pigment does, and that's exactly why they're the easiest to underestimate. Solvent's whole job is to leave. Additives go in at well under one percent. Get either one wrong and the failure lands right where you can't hide it: the dried film in front of a customer. The first half covers solvents: why evaporation rate drives film quality, why formulators blend fast, medium, and slow solvents instead of using one, what changes when water is the solvent, VOC in plain language, and flash point, the one solvent number you never ignore. The second half covers additives: the main categories (dispersants, defoamers, rheology modifiers, biocides, UV stabilizers, driers), the golden rule of dosing, and why fixing one defect with "more" of something often creates a new one. Two worked troubleshooting examples, orange peel and pinholes, show the same habit from the last two episodes: raw material, formulation, application, and process are separate suspects. Check all of them before you point at one. This is Episode 12 of the Coatings Industry Training Series, based on Book Two: Raw Materials for Beginners.