Dip coating is a simple old way of depositing onto a substrate, especially small slabs and cylinders, a uniform thin film of liquid for solidification into a coating. The basic flow is steady, and in it film thickness is set by the competition among viscous force, capillary (surface tension) force and gravity. Thickness and uniformity can be sensitive to flow conditions in the liquid bath and gas overhead. The faster the substrate is withdrawn, the thicker the film deposited. This can be countered by using volatile solutes and combining rapid enough drying with the basic liquid flow. Then the physics grows more complicated, theoretical prediction of process performance more difficult, and control of the process more demanding. Outside product R&D labs it is far less often used in precision coating manufacture than a variety of premetered coating methods.
Spin coating is a more recently developed way of getting onto piecemeal substrates, especially small flat disks, a uniform thin liquid film for the same end. The basic flow is unsteady radial drainage in which centrifugal and viscous forces so compete that ordinary (Newtonian) liquid of constant viscosity tends toward a uniform film that grows ever thinner ever more slowly. Volatile solvents are commonly used because conditions can often be found that adequately separate thinning by spin-off from later thinning and solidification by drying. Thickness and uniformity, today theoretically predictable, are sensitive to speed, gas conditions, and rheology of concentrating, solidifying liquid. For the rheology of photoresist coating in microelectronics, spin coating works well. For that of suspension coatings in magnetic disk technology the process demands more careful control; actually it is often modified.