When going down to bare metal you should be using an etching primer. I
found an old email on the list regarding some painting questions and it
follows this. If primer is left exposed it will absorb moisture and surface
rust might not be visible immediately but will eventually show up, as
primer is very porous. Usually folks use the POR 15 on chassis and other
items, not so much for body panels prior for prepping prior to painting. We
all think we're fast when it comes to bare metal, but surface rust starts
much quicker that you thing, maybe not visibly but it is there. I know a
lot of vehicles get what is called a skim coat of bondo to get the flat
body panels smooth, its much quicker to get bondo completely straight as
opposed to metal.Here is the email on the subject mentioned. --wayne
The sequence of painting these days is generally as follows: (After all the
metal work is done)
First is an "etching Primer". This stuff goes on real thin (A single coat
is closer to a haze than a coat of paint). This etches right into the metal
for maximum protection. You can use polyester body filler over an etching
primer if you find you have scratches that you don't feel a primer/surfacer
will fill.
Second is the primer/surfacer. This is applied and is used to build into
small scratches. A Primer/surfacer is porous so that a top coat will adhere
to it nicely. If this goes on right over the bare metal, you have just put
on a very porous coating and you could get moisture past this stuff. This
is a multicoat process with "wet or dry"sanding between each coat. Before
you top coat, the primer/surfacer needs to be free of all defects. Any
Scratches at this stage will show in the final stage.
Some systems put a last sealer step in here before the top coating. This
step is generally NOT sanded as it is a sealer.
Finally the top coating is done. The top coat can either be a single stage
paint or a base coat/clear coat set up. Metallic paints are generally base
coat/clear coat. The base coat is actually rough looking allowing the
metallic paint to catch light easily. The clear coat goes over this to give
it both depth and protection with a nice Smooth finish. Clear coats can be
buffed out as well as other paints. Just be careful not to wear through the
clear coat, or you are into a blend repair and a lot more work than just
another coat of paint like when you are using a single stage top coat.
Most paints have two or three parts mixed together and put into the gun like
a cocktail per application. To get the right proportions they sell
disposable plastic mixing containers with graduations marked on the side for
various standard proportioning systems Like 1:1, 2:1, 3:1:1 etc. Once mixed
these components begin to interact with each other. The instructions will
tell you what the "pot life" is. After the "pot life" of the mixture has
been reached, its either self hardening from the chemical reaction or the
reaction is over and the liquid is not active enough to bond properly if
applied.
Don't mix and match across product families. Stay with the manufacturers
system. This assures proper adhesion layer to layer without "surprise"
interactions. Make sure each layer is clean and ready to be coated per the
instructions. Some finishes, like some primer/surfacers can be coated with
the next step as long as no more than XX hours are allowed to elapse between
coats. If more time elapses, you make have to prepare the surface with a
prep solvent before moving to the next step.
While you are at the paint store, ask about spray systems. That can get
even more long winded than this response.
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