"Buick Trademark(s) used with the written permission of General Motors"
Home Forums Garage Classifieds Reference

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.
Lets 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 base coat 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.

- Very thin, keeps down film build
- Cost effective
- Fast application
- Non-sanding
- Super high corrosion protection.
- 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.


- Easy to apply, smooth, easy to sand
- Some can be applied over plastic filler (not that you need it over 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


- 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.


- Easy to apply, and sand.
- Applies smooth.
- Fills well with minimum of shrinkage


- 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.


- 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 .


- 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 or mils per coat while polyester can give you as much as 4 to 6! Because of its 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.


- VERY high filling
- Low cost
- Very high texture
- Harder to sand than a urethane
- Possible need to purchase a large gun to shoot it.


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.


- Isocyanate free
- Smooth, easy sanding
- Good filling
- 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.


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, Its only as strong as its 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 base coat 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 cant 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!


Brian Martin