“Two coats” sounds precise, but coat count alone does not tell you how much coating will be left on the surface. Coverage rate, porosity, application method, wet-film thickness, dry-film thickness and product solids determine the actual film.
A Coat Is Not a Standard Unit
A heavily loaded roller pass and a thin sprayed pass can both be called one coat. A porous block wall can absorb more material than a sealed smooth surface. Rough stucco has more actual surface area than the square footage measured from the building dimensions. For high-build, roof and protective coatings, the target film thickness may matter more than the conversational number of coats.
Use the Product Data Sheet
Sherwin-Williams’ current Duration Exterior Low Lustre data sheet is a useful example. It publishes a coverage range of 250–300 square feet per gallon tied to approximately 5.3–6.4 mils wet and 2.2–2.6 mils dry, and it notes that drying depends on temperature, humidity and film thickness. The same product family also distinguishes one-coat repaint use from two coats on new bare substrates. Those numbers belong to that exact product—not to exterior paint generally.
Why This Matters in a Quote
If one contractor budgets five gallons and another budgets ten for the same area, the difference may reflect product coverage, substrate texture, number of coats, expected waste or simply a different application plan. The proposal does not need to become an engineering specification for ordinary house painting, but the material quantity should make sense when compared with the manufacturer’s spreading rate.
For Specialty Coatings, Ask for Film Build
Roof coatings, epoxies, urethanes, polyaspartics and some elastomeric systems are commonly specified by wet or dry film thickness and total system build. In those jobs, “two coats” without a target coverage rate can conceal a large difference in installed material.
Practical Questions
- What exact product is being applied?
- How many coats does its current data sheet require for this substrate?
- What spreading rate or square feet per gallon is the bid based on?
- Does the surface texture or porosity change that estimate?
- For a high-build system, what wet/dry film thickness is targeted?
- How will the contractor verify film build if the manufacturer requires it?