Sealing a model with the proper clear coat products is an important part of the build-up process and, in many cases, sealing happens more than once and often with more than one kind of sealant. A sealant can both protect a model kit's paint job, and help prepare a surface for decals. Flat coats and gloss coats, including acrylic floor waxes, are the most common types of sealants used in model construction.
Flat coats, also known as "matte" or "dull" finish applications, leave a kit without a shine. (See Reference 1) Flat coats are commonly available in aerosol cans and liquid form in hobby stores and model aisles of craft stores and are most commonly applied in military and railroad modelling. Additionally, animals, people and buildings typically require a flat finish as does the underside of automobile models. They should not be applied to chrome or clear plastic parts.
Gloss coats are most commonly utilised in automobile modelling, though they may also be applied to commercial aircraft and any surface that would be shiny when new, assuming you are modelling something new. (See Reference 1) A new fire hydrant, for instance, is shiny, but few look shiny on the street. Water features also benefit from at least one application of gloss coat. Again, chrome and clear plastic do not fare well with gloss sealants.
Acrylic Floor Wax
Liquid acrylic floor waxes are commonly used in model building. One application is to add a sense of moisture to eyes, lips, gums and teeth on figure models. Floor wax is also an excellent way to prepare a surface for decals, allowing them to stick better or to improve clear plastic parts. (See Reference 2 & 3) They can be sprayed through an airbrush or brushed on and flat coats can be applied over the top to finalise a kit if necessary.
Natural Metal Finishes
Natural metal finishes were common on aircraft and some railroad engines and rail cars in the 1940s and 1950s. Neither gloss nor flat coats will look convincing on such models. Instead, specialised paint products designed specifically to adhere properly to plastic and provide a proper buffed silver appearance are used. (See Reference 4) Metal foil may also be applied in some cases for the top coat. (See Reference 5)