High viscosity is better for build-up, patching for coarse aggregates, thick adhesives, etc. For thick castings or whenever curing is conducted at higher temperatures. Indeed, often surface preparation is the longest and most expensive part of the project. This subject should be treated separately, but a mention here is useful. Sandblasting, for instance, should be the first choice for metal, concrete and most surfaces.
Several treatments are available for metals, plastics, etc.
Oils, grease, waxes, fats, soaps, lubricants and many other materials prevent adhesion. They must be removed with solvents, detergents or other chemicals known to be effective with the interfering materials. In all cases, surfaces must be chemically clean and dry. Often, part of the epoxy to be used can become its own primer by diluting it with an appropriate solvent and using this diluted portion with brush, roller or spraygun to prime the surface involved. Others, like fiberglass or other fibers, are used for strength.