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.
"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.
"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".
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.
"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.
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.
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.
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.
"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.
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.
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!