Coating, Application process

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Description

A coating is a thin layer deposited or applied on the surface of an object (substrate). The primary purpose of applying a coating is to improve an objects critical properties and to create a protective barrier against deterioration of the surface. Secondary purposes may be aesthetic. Coatings may be applied as liquids, gases or solids e.g. powder coatings. A major consideration for most coating processes is to control coating thickness and adhesion. Methods of achieving this range from a simple brush to expensive precision machinery. Thus, the goal of this table is to compare the advantages and disadvantages of some of the techniques.

Note: For information on the types of materials used, see Coating.

Coating Application Processes

Coating Process	Description	Advantages	Disadvantages	Typical Materials Used
Anodizing	An electrolytic passivation process where the metal surface is converted into a durable, decorative, and corrosion-resistant oxide layer.	Coating grows from the substrate itself rather than being applied over it; offers unmatched aesthetic durability.	Limited primarily to aluminum and its alloys; colors can fade under prolonged UV exposure; can be expensive.	Aluminum, titanium, magnesium, and niobium.
E-coating (Electrophorectic)	Parts are dipped into a bath of liquid (water-based) paint electrified to cause the coating to deposit evenly on the metal surface.	Offers superior edge-to-edge coverage and targeted, uniform film thicknesses.	Upfront equipment costs are high; usually limited to conductive materials (metals); requires dip tanks; primer required	Acrylic latex
Painted Coating	A continuous process where liquid coating is transferred from a container	Simple equipment- Good for small areas or touch-up- Low overspray;	Mechanical complexity; requires precise fluid mechanics (viscosity control) to avoid streaking, ribbing, or uneven thickness.Slower for large areas- May not achieve uniform film thickness	Alkyds, Acrylics, Latex, Epoxy Esters; Liquid coatings (varnishes, lacquers, plastisols, and water-based adhesives)
Powder Coating	Dry, finely ground powder is applied electrostatically to a grounded part, then melted and cured in an oven.	No solvents; 100% solids; creates a thicker, tougher, and more uniform finish than liquid paint; excellent durability	Difficult to achieve thin coats; color changes require time-consuming equipment cleaning; requires heat-resistant substrates.Requires oven curing; not suitalbe for large strucures; special equipment needed	Thermoset/thermoplastic powders (Polyester, Epoxy, Acrylic, Polyurethane).Epoxy polyester, polyurethane powders
3-D Printed Coating				PLA, ABS, Metal; Ceramic (Cerakote), epoxy resins (XTC-3D)
Spray, conventional		Fast coverage, Uniform film; Good for complex shapes	Overspray and drifting; Requires ventilation; Not ideal for outdoors	Alkyds, acrylics, epoxies, Urethane
Spray, thermal	Metallic or ceramic material is melted by combustion or electric arc, then sprayed at high velocity onto the prepared substrate.	Excellent for wear and corrosion resistance; high deposition rate; no size limits for the part; immmediate use after application	High equipment noise and heat; requires highly skilled operators; potential for poor bonding if the surface is improperly prepared; porous substrates needs sealing prior to coating	Metals (zinc, zluminum), metal alloys, ceramics, and some plastics.
Tape wrap, Shrinkwrap	Physical application surrounding a substrate	Used widely to protect from moisture and dirt, cost effective	Susceptible to mechanical damage; may not be air-tight; shrink to form requires heat	Polyvinylchloride, polyethylene, polyolefin, butyl, ethylenepropylenediamine monomer(EPDM), nylon, glass fibers.
Thermoplastic	plastics dissolved in solvents that cure by solvent evaporation	Process is faster		Vinyls, chlorinated rubbers; latex emulsions
Thermoset (oxidation)	Liquid coatings usually dissolved in solvents that cure by crosslinking with oxygen. Can be accellerated with metallic salts	The process is slow for solvent evaporation and curing	Oxidation continues over the lifetime of the coating casuing cracking embrittlement and deterioration	Drying oils, catalyzed epoxy resins, polyurethanes, vinyl ester
Vapor Deposition	Vaporized materials are deposited on a substrate molecule-by-molecule inside a vacuum chamber.	Creates ultra-thin, precise, and highly dense protective layers; no limitations on substrate materials.	High processing costs; geometric line-of-sight limitations (recessed areas may not be coated easily); limited coating thickness.	Titanium nitride (TiN), chromium nitride (CrN), diamond-like carbon (DLC), and various metals/alloys.

Resources and Citations

• Gilbert Gedeon, 'Coating Types and Selection, Course for Professional Engineers, CED Engineering Link