Single layer metallics - these have no separate clear lacquer
Thin the remaining paint mixture a little further than previously, about an extra 5-10% thinners only. (Not activator if using 2-pack) Once the previous coat has been left to flash off (but not dry), usually about 10mins depending on thinners, apply two or three lighter coats with the gun held a few inches further away from the panel, with very little flash-off time between. These coats should give an even metallic surface, whilst still being able to soak in to the previous layers, giving a smooth surface.
If using cellulose paints, don't be tempted to apply a finishing coat of clear cellulose or blending clear, as they do not "weather" at all well. This limitation does not apply to acrylic or two-pack lacquers
Do not wet the surface with thinned paint, otherwise blotches will reappear or it could sag. The surface when dry will be slightly matt, particularly if cellulose paint was used, but should be smooth enough to compound to a good gloss. (Single layer metallics do not have the full deep gloss of a lacquered finish) Do not compound or cut back heavily with single-layer metallics, as over-use of compound or abrasive polishes will bite in to the aluminium flakes in the paint, leaving a "greyed" effect which can not be removed by further polishing.
Metallic basecoat systems - clear lacquer over base
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
Pearlescent metallic colours
A lot of modern metallic colours now have a "pearl" effect, which shows a greater degree of colour change when viewed from different angles(flip/flop) compared to normal metallics. This is achieved by replacing some, or occasionally all, of the aluminium in the basecoat formula with mica flakes. The mica itself is virtually transparent and acts as microscopic lenses producing a prism effect within the paint.
Normal metallic colours which contain "pearl" are applied in exactly the same way as a standard metallic, but may need extra coats as they can be poor to cover. A coloured groundcoat (undercoat) may also be specified to ensure a uniform appearance.The mica pigments also tend to be expensive, so be prepared to pay more for pearl colours.
Some pearl colours are composed of three distinct layers, firstly a coloured basecoat is applied (not to be confused with a groundcoat). This is followed by a pearl effect basecoat, applied in the usual way, which is then finished with lacquer. This type of system is commonly found on white pearl effect cars, but is becoming more common with other colours
A further variation of these systems can include a tinted (coloured ) lacquer used as the final coatings.
Obviously with this type of finish colour variations can be produced by the application of more/less lacquer, as well as the metallic/pearl layers beneath.