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Aerosol v Spray Gun
courtesy Jon Mitchell (Independant Porsche Mechanic)
OK, down to basics.
Gloss from the gun/can is achieved through even application of paint.. for
it to gloss from a gun, or can, the following has to be correct.
1. Distance between the nozzle and the surface. too far and the paint
will begin to dry before it hits the panel (dry looking and dull, a little
like sandpaper).. too close and too much paint will be applied too
quickly... Air (propellant) pressure also comes into this as higher pressure
air (propellant) will cause the paint to move at higher speed, which
effectively reduces the distance, too low a pressure and the paint will not
be atomised into fine molecules which would result in paint droplets hitting
the panel.
2. Coat thickness. Too thin a coat and the paint molecules will not flow
into one another, which again will cause a dry dull sandpaper Finnish... too
thick a coat and the paint will run, or as the paint dries, a skin will form
on the paint, which will cause the encased paint to dry more slowly, this
causes the famous orange peel effect (dimpled or hammered look). As a rule,
coat thickness is determined by the speed of movement of the gun/can...
moving slowly causes more paint to be applied, moving fast causes less.
3. Preparation. For a good gloss, the surface that the paint is applied
to must be perfectly smooth and without texture, while still having a
microscopic surface finish keying (flatted of with 1200 - 800 grit wet
paper)
Now with a can, the problem is that the spray pattern is circular, which has
a result, that as you spray in a line, the centre of the line picks up more
paint... the effect of this is that the paint is either too thick in the
middle and too thin at the sides (gloss in the middle, dry sand paper look
at the outside.)
Another problem with a can, is that the air is not at a precise regulated
pressure. So when you start, the pressure (therefore delivery rate) is high,
and as the can empties, the pressure (delivery rate) is low... so you have
to watch your distances and application speed with cans because as the
pressure in the can decreases, so the delivery rate will decrease.
There are a few things you can do to improve application using cans, here
are some tips.
1. Paint flows and atomises better when warm, so warm up the cans in a
bowl of warm (not boiling, but hot tap temperature) water before you use
them, if your doing a large area, that's going to use more than one can, or
more than one coat, then keep the water warm, and keep your other cans
standing in the bowl while your using another, or between coats. This also
has the effect of slightly increasing the pressure of the propellant which
is desirable.
2. Paint not only flows from a can better when warm, but the paint, when
it hits a warm surface, will flow better too, with the molecules more happy
to flow into each other when warm, increasing the chances of a good gloss.
Another side effect of a warm surface/panel, is that the paint will dry
faster once applied, and more importantly dry evenly throughout its depth
(avoiding orange peel)... so warm the panel a little with a hair dryer or
hot air gun before you shoot the paint... but as well as this, while
spraying, keep the can in one hand, with the hot air gun/hair dryer in the
other following the action of the spay gun... but keep the heat source
moving or swirling to avoid a build up of heat in a localised area...
another idea is to keep the heat setting on a hot air gun to a minimum, and
on a hair dryer to a medium setting.. otherwise you will blister the
previous or fresh paint..
3. Shake the cans really well, for as long as you can before use, and
give them couple/few shakes at the end of each pass to keep the paint
mixed... this is not just to keep the colour mixed, but to ensure that the
thinners in the paint is also well mixed with the colour... all too often if
you do not do this, you will find that the paint will apply too translucent
when you first use the can, and become thick by the time the can is empty...
also the paint will be less predictable, which will make judging your
distances more tricky.
4. If your going to use a can, try to buy ones with a inserted nozzle
that provides a more fan like pattern than the normal conical/circular
pattern from your average Holts style can.
5. If your using a gun, a trick is to place a few (ultra clean) ball
bearings in the paint pot on the gun. this allows you to swirl the gun after
each pass so the paint stays mixed by the action of the ball bearings
pushing through the paint and stops the paint solids from settling to the
bottom of the pot (along with keeping metallic particles on a metallic job,
well mixed and applied in equal amounts from pass to pass and stroke to
stroke).
6. Pattern is important, paint in straight lines, with the can/gun at a
right angle to the surface AT all times, each pass should overlap the
previous one by 25 - 50%.
If your using a standard nozzle on a can, do one coat on a horizontal axis
(as overlapping stripes painted left to right), the next on a vertical one
(again overlapping stripes going from top to bottom)... with a gun, stick to
horizontal stripes.
7. Paint is more likely to run on corners for several reasons. So its a
good idea to paint any corners as your first coats... with corners like
wheel arches, or other edges, rather than this first coat being applied in
straight lines, it should be applied in strokes that follow the lines of the
car, but again overlap your bands if your painting more than one pass. On
later coats of the flatter areas, spray your bands regardless of the corners
and treat them as if they were not edges at all, just be cautious of not
applying to much to them on later coats.
COATS:
One of the most important things to know about coats is "flash times", this
is the amount of time you leave between coats... when using a spray gun and
2k paint, flash times need to be no more than 15 mins but more than 5
mins... with cellulose. the flash times can be up to doubled... with a can,
follow the rules for 2k paint, as your coats will be thinner! The reason for
flash times, is to allow the thinners to evaporate, but not allowing the
paint to fully harden (it needs to be just very slightly tacky) between
coats, this allows the following coat/coats to bond to the previous coat...
if the previous coat has allowed to dry fully, the next coat will not stick
to it, and so any fully hardened coat should be lightly wet sanded with 800
grit wet paper, so the next coat can key to the previous one. with this
technique, the sanding needs to just "Knock off" the gloss.. so you know you
have sanded enough when you wipe the panel with a paper towel and it looks
dull.
With solid single stage schemes (no lacquer) with cans you should use about
5 coats minimum, with a gun that could be cut down to 2 or 3.
With twin stage colours, the base (colour) coats must be thinner, should not
be glossy otherwise any metallic content (tiny metallic particles) will sink
into the paint and be lost (leaving the base non metallic). On these twin
stage schemes, the lacquer should be applied as with any other coat, meaning
that it needs to be applied while the base is still slightly tacky,
otherwise later you will be a victim to the famous lifting lacquer syndrome.
A word of warning with lacquer (mostly from cans), do not apply the coats
too thick, otherwise it will go milky... you need to use JUST enough for a
gloss on each coat.
A tip with lacquer, is that the more coats you apply, the deeper the finish
will look... imagine the lacquer is a coat of glass, thick glass will look
like a deeper finish than thin glass, the same is true with lacquer.
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Originally Posted by jaye944 
