Most basecoats use only thinners, but occasionally activator may be used, e.g. if applied over plastics. Check with the supplier. Transparent colours may need a coloured undercoat, called a "groundcoat". Again, your paint supplier can tell you if one is needed for a particular colour. Groundcoats and solid-colour basecoats are applied as normal coats as per ordinary solid colours, observing correct flash-off times.
With their greater covering power, basecoats will usually only need two or three coats. Apply in even coats until the underlying repairs, etc., are blanked out, allowing the manufacturers recommended flash-off time between coats. For the final coat before lacquering, attention must be given to the manufacturers instruction sheets, for the following reasons;
Some basecoat metallic colours are applied as "wet" coats, and dry to give an even metallic finish without further attention. Other types may require the application of one or two cross coats (lighter coats as used for single layer metallics) to produce an even, cloud free finish prior to lacquering. Choosing the correct method for the material in use is vitally important especially if colour matching adjacent panels. In all cases the basecoat is allowed to flash-off, but not dry before lacquer is applied. Remember, any stripes, dirt or other defects in the basecoat will be effectively `locked' in once lacquer is applied. Special basecoat "tack-rags" are available for use between coats and before applying lacquer, to minimize dust problems. Obviously the surface must be touch dry to use anything like this
