Paint technology basics
by Brian Martin
There are many different types (or more correctly,
technologies) of products you can use in the restoration or repair of your
vehicle. Some have a variety of uses while others are very limited with only a
few of specific uses. Proper choice of products can help you get the job done
faster and/or help with the longevity of the repair. Let's start with some basic
definitions. I couldn't possibly know every paint manufactures terminology or
product use. These are generalities and should be used as a guide only to then
read the tech sheets of the products you have chosen for proper use. These tech
sheets can be found at the jobber and are given away free. Or most manufactures
have them on line, USE THEM. They are a wealth of information and can save you
many headaches. You don't need to read every word in the mind numbing text, they
usually have a "product at a glance" or something like that will cut
to the chase and give you what you need.
Basic terminology's:
"Solvent" is a generic term and refers to any
"reducer", "thinner" that is used to reduce the viscosity
("thickness") of a product to aid in spraying or applying. It could be
acetone, lacquer thinner, urethane reducer, a special "basecoat"
reducer, water, alcohol, etc. These solvents ARE NOT INTERCHANGEABLE; each
product MUST be used with the specific solvent recommended by the manufacture.
"Etch primer" an acid containing primer. "Primer" a product
that can be applied to bare metal "Surfacer" (or "primer surfacer")
A primer that has "body" or solids and is used to fill imperfections
and provide a film thickness to sand or block a surface to a smooth base for
paint. "Sealer" a non-sanding product that is applied prior to
painting. "Primer-sealer" A sealer that can be applied over bare metal
and then top coated without sanding. "Flash time" the time you allow
the solvents to evaporate out of the film you have applied.
Basic technologies
"Single component" or RTS (Ready To Spray).
This is a product that uses no additional components. Just pour it from the can
into your gun and shoot. Examples are: Some plastic adhesion promoters and
primers and even some top coats like vinyl colors.
"1K" This is a product that uses no hardener,
catalyst, activator, etc. It may have an added solvent, but no hardener or
activating reducer. 1K products like RTS dry with the evaporation of solvents
and are soluble, meaning that they are could be wiped off with a rag soaked with
lacquer thinner. They could in THEORY be scraped off and put in a can with
solvent and stirred back to a sprayable condition. Of course ALL RTS products
are 1K. Examples: All lacquer products, some synthetic enamel products, and some
acrylic enamel products. Because of the low VOC regulations the 1K product
options are getting scarce, with most limited to "specialty products"
like adhesion promoters.
"2K" or "Two component" is any
product that uses a hardener, activator, catalyst, etc. It may or may not use a
third component in the form of a solvent. 2K products don't "dry" like
a 1K. The 2K product "cures" by molecules linking together to form a
whole new compound. Most high quality 2Ks are insoluble after a full cure and
will not soften when exposed to solvents like thinners or gas. Examples are
urethane under coats and top coats. Epoxies, ISO free products that use a
hardener, etc.
Basic tip, ALL 2K products should be mixed as accurately
as possible. As a rule 2K products need a minimum of 55 degrees to cure with an
ideal minimum of 65 degrees. MIX THEM AS DESCRIBED BY THE MANUFACTURE. They have
spent hundreds of thousands, possibly millions of dollars developing the
product, they WANT it to work as BEST it can. Do as they say, don't become a
"Junior Chemist".
Types of products and their uses:
Etch primers (some are 2K)
"Wash" or "Vinyl wash" are for bare
metal applications for the ultimate in adhesion and corrosion protection. They
are very low in solids with next to zero filling qualities. Some are even semi
transparent. They are usually not to be top coated with paint. You apply them to
aid in adhesion and corrosion protection under other undercoats such as epoxy or
urethane primers.
Benefits:
- Very thin, keeps down film build
- Cost effective
- Fast application
- Non-sanding
- Super high corrosion protection.
Disadvantages:
- Some have a very small re-coat window
"Etch primer" (some are 2K):
Typical "etch primers" have much more solids
and body than "wash" primers. They are more forgiving than
"wash" primers, one thing being a much longer re-coat window. They are
basically used to aid in adhesion and corrosion protection as with
"wash" primer. You would choose "typical" etch over
"wash" if you have some paint or plastic filler as a substrate along
with the bare metal. Some brands have a recommendation to apply top coats over
it also. This could be very useful in a money saving or time saving is
important.
Benefits:
- Easy to apply, smooth, easy to sand
- Some can be applied over plastic filler (not that you need itover the plastic
filler, but if you have some, it is nice to not have to go around it)
- Some can be top coated, which can be a big time and money saver.
- VERY cost effective
Disadvantages:
- Added product to buy and apply.
IMPORTANT basic! If you have used ANY metal treatment or
"conditioner" read tech sheets carefully for compatibility . The acid
in the metal "treatment" or "conditioner" can attack the
acid in etch primers and it can LOOSE adhesion from the metal!
Urethane primer (2K) Urethane primer is the most common
primer used in auto body and restoration by far. It has good solids and fills
well. It is easy to sand and can provide you with a perfect body when blocked
properly. Care should be taken when applying it as to not use too much. It can
shrink when applied too heavy. It is the best all around primer for applying
over plastic body filler and for surfacing your work. If used properly it
provides the proper film thickness under top coats and is the perfect substrate
for bs/ss and SS.
Benefits:
- Easy to apply, and sand.
- Applies smooth.
- Fills well with minimum of shrinkage
Disadvantages:
- Contains Isocyanates.
- Should always use an etch primer under it.
Epoxy primer (2K)
Epoxy is a good corrosion fighter. It is has a very
sticky resin and will provide good adhesion to MOST substrates. It typically has
poor filling and sanding qualities (that sticky resin makes sanding difficult) .
It is ideal for use as a "primer/sealer" on bare metal that requires
no surfacing. Perfect for frames and components, radiator supports, items that
are sandblasted and you only need to prime and paint. You use it as a
non-sanding "primer/sealer" and then paint right over it.
Benefits:
- Good chip resistance (it isn't as hard as a urethane)
- Perfect for a "primer/sealer" over bare metal.
- Etch primers can skipped because of its excellent adhesion and corrosion
properties. (although for maximum corrosion protection apply a wash etch under
the epoxy)
- Provides good base under plastic body fillers (skip the etch if you plan on
using plastic filler over epoxy) - Epoxy has no isocyanates .
Disadvantages:
- Poor sanding qualities
- Poor filling
Polyester primer (2K)
Polyester is a very specialized primer used in very
small amount in most shops across the country. But when it is needed, it does a
job like no other. Polyester has a huge solids content and will fill 80 grit
scratches in one coat or 36 grit in two or three! Urethane for instance provides
about 1/2 or 3/4 mils per coat while polyester can give you as much as 4 to 6!
Because of it's high solids, it shrinks very little. It is basically like
spraying polyester putty. Look for a manufacture that has a recommendation to
apply etch primer under it. I see NO reason to use polyester on a straight
panel. It is for use only when you need some serious filling and surfacing.
Benefits:
- VERY high filling
- Low cost
Disadvantages:
- Very high texture
- Harder to sand than a urethane
- Possible need to purchase a large gun to shoot it. - "ISO FREE" (2K)
"ISO FREE" is a urethane type primer but
without the harmful isocyanates that a urethane contains. The problem is ALL
refinish products should be used with the same care and concern for your health
and others. ISO FREE is like "low tar" cigarettes, don't kid your
self, it is still VARY harmful.
Benefits:
- Isocyanate free
- Smooth, easy sanding
- Good filling Disadvantages:
- You need an etch over bare metal before it.
Basic tips... Etch primers can be skipped on spots of
bare metal smaller than a dime or so when using all primers listed. Most
"quality" 2K primers need NO sealer before top coating with bc/cc or
SS when applied properly.
Sealers
All RTS or 1K sealers should be reserved to VERY low end
jobs to save money. They do NOT offer the benefits of a 2k, PERIOD.
Reasons to use a sealer:
- Makes up for "some" poor preparation
- Provides a uniform color for better coverage when you apply paint.
- Helps with providing a uniform substrate for paint.
- Helps provide a better substrate when painting over a 1K primer.
- Can Help with "covering" poor prior repairs
Under collision repair conditions a shop may use sealers
on every job as an "insurance" protection against problems. In a
restoration environment where complete panels are primed with a 2K there really
is no need to use them. If you have chosen to use a sealer there now are a few
more choices to make. First, you need to decide what kind of sealer to use. As I
mentioned in the beginning, RTS or 1k could be used to save money. Why put a 1K
sealer over your 2K primer (I hope you are using a 2K primer) then apply a 2K
top coat? It is like the old saying, "It's only as strong as it's weakest
link". If you use a 1K sealer in this fashion it is like replacing a link
in your tow chain with a nylon tie!
With 2K there are a few options, epoxy and urethane
being the most popular. I don't feel that there is a huge difference in the two
as far as how they apply or work. Epoxy is more forgiving with sensitive
substrates. It really comes down to what you feel more comfortable with. The
epoxy has no isos so that would be one reason to choose it. Now that you have
decided what sealer to use you have to decide on what application. Most sealers
give you the option of a "wet on wet" (or very close to it) or a full
"barrier coat" application. The difference being with "wet on
wet" the sealer is applied and then allowed a short flash time before the
basecoat or SS is applied. A "barrier coat" is where the sealer is
applied, then allowed to cure or at the very least to totally flash. This allows
the sealer to become a barrier so the solvents from the color coat can't
penetrate it and attack the substrate. The barrier coat procedure allows for he
sealer to do MUCH more of what you choose to use a sealer for in the first
place. The choice is made taking into account a few factors. How sensitive is
the substrate? Or, how aggressive are the solvents in the color coat that you
are applying? If it is very hot weather and you are using a slow solvent in the
color coat to help it lay out, you may choose to use a sealer because you know
that the substrate is sensitive and the slow solvent will attack it. Well, that
is about it for the basics, have fun!