Well Charlie, seems no one wants to tackle this question. You are really asking about the binders that are used to produce paints.

I can't think of any polyester based paints that are being sold for use on our models. Most polyester paint systems are the powder coating systems that require heat to fuse the coatings. There are some solvent based polyester/alkykd paints that are blended with silicones to produce flexible corrosion resistent coatings. Not sure that they are very fuel resistant.

Urethanes are commonly used for model airplanes. Two part (crosslinked) automotive paint system are usually urethanes.

This website does a nice job of describing paint systems in general   

http://www.p2pays.org/ref/01/00777/alternat.htm


and an excerpt from this site taht pretty much answers your question...

A Few Words About Binders
Binders are chosen based on what physical and chemical properties are desired of the finished film. In general, metal coatings are dominated by alkyds; however, water-based acrylics, epoxies, polyurethanes and polyesters also are used for certain applications (MPCa, p. 34).

Acrylics and alkyds contain suspended polymer particles. These materials produce a shiny, hard finish that has good weather resistance. Alkyds are made from chemically modified vegetable oils and are relatively low in cost. They are easily modified in order to change the properties of a paint. They also can react with other chemicals in the curing process to change the finish. In ambient conditions, they react with oxygen to form cross-linked films, making them functional for a wide variety of applications. Because of the versatility and moderate cost, they are considered "general purpose paint" (KSBEAP, p.4). Both acrylics and alkyds are widely used for farm equipment and industrial products that require good corrosion protection at a moderate cost. Silicone modification of these resins improves overall weatherability and durability. These finishes are often used on space heaters, clothes dryers and barbecue grills.

Urethanes combine high gloss and flexibility with chemical and stain resistance. They are also characterized by toughness, durability and corrosion resistance. They require little or no heat to cure. These materials usually cost 2 to 5 times more than other paints so they are often used in applications where high performance justifies the cost (KSBEAP, p.4). Typical uses are on conveyor equipment, aircraft radomes, tugboats, road-building machinery and motorcycle parts.

Urethanes are produced by a reaction between isocyanate and alcohol. The components can be mixed in a "pot" prior to application or can be mixed in the atomizing portion of the spray gun. Once mixed, urethanes have a limited "pot life," which is the amount of time the components can be mixed before crosslinking occurs. Often, pot life can be adjusted to meet process requirements; typical ranges available commercially are a few minutes to 16 hours (KSBEAP, p.5).

Epoxies provide excellent water and chemical resistance. They have better adhesion to metal substrates than most other materials. Epoxies are attractive economically because they are more effective against corrosion in thinner films than most other finishing materials. They are often used as primers under other materials that have good barrier properties but marginal adhesive characteristics. They can be formulated in a variety of ways, from one-component formulations requiring elevated temperature curing to two-component systems that cure at or below ambient temperature conditions. Epoxies lose their gloss from ultraviolet exposure but the damage is rarely structural (KSBEAP, p.4).

Polyesters, are similar to alkyds in chemical structure but require heat to cure. They are used extensively in powder coatings. Polyesters, such as Nylon 11, provide an attractive appearance as well as protection from chemicals, abrasion and impact. Nylon coatings are used on office and outdoor furniture, hospital beds, bending machine parts and building railings. Heavier coats can be used to protect dishwasher baskets, food-processing machinery, farm and material-handling equipment, and industrial equipment such as pipes, fittings and valves (MD, p. 703-704).

Other Binders, such as silicones have high heat resistance and superior resistance to weather and water. They are used alone or blended with acrylics or alkyds. Vinyls are another binder that can have a wide range of flexibility. They are used extensively in marine applications, interior metal can liners (e.g., polyvinylchloride), or structural wood finishes (e.g., polyvinylacetate) (KSBEAP, p.5).

A little info Please!
 I need to know the difference between polyurethane and a polyester finish.
 Ive a feeling I can get the answer here.

Hi Charlie

Dick gave an excellent  response, In addition I will add this, In Model airplanes  people have been using  what they  call polyester finishes for  on and off  about  35 years, Bob Hunt used to do this, What they call a  polyester finish  is  simply a  Fiberglass resin  base. To do this  many used Polyester resin brushed onto the airplane  and  taken off  as  soon as  possible  with many differant methods, many  used  toilet paper  or  paper  towels  to  soak and  wipe  the resin off, this  took most of the weight back off the airframe  and  when dry  it  left a  hard  polyester (fiber glass resin) finish that you could  sand smooth  and  apply dope  or whatever. Most did not  use  any paper (silkspan) on any of the solid wood surface, this was used  widely back in the late 70s  early 80s  on  foam or  fully sheeted wings.

The Poly U finish  you hear about  is  just that polyurethane paint. either  1  or  2  parts . these  finishes are  like  an  Imron finish, and  quite heavy, So  most  people  would use  dope  or  Lacquer, then when  cured out  they would  top coat a final clear coat of  Poly U  catalysed 2 part  and  have  a  much lighter  finish  that was  totally fuel proof,  you could  clean this  with  dope thinner  and  not hurt your  finish, many people  use this  method  today.  Bill Werwage for  one.

I will also  mention  that there is  also  a  " CA  finish"  many people  will coat  the wood  surface  with  CA  just  like  they did  with  Polyester  resin  and  sand  smooth, using  it  as  the base  for  there  finish.

Go  fly something   :-)

Randy