Artists will need polyester and polyurethane resins when it is time for clay modeling and other forms of casting. These resins are ideal since they can cure and form a hard cast. They can also come off as glass, and shine through when they have been sanded and buffed. But you need to deduce which one to use. To make a better choice, you need to know how different these resins are.
Polymer resins are not the most expensive of the two. They also rely on catalysts or an accelerator for it to work properly. They however produce a toxic odor, which makes it mandatory for an artist to wear a protective gas mask when working with it. You also need to work in a properly ventilated studio. You also need to add in some form of reinforcement, preferably fiberglass. In future you can expect them to develop spider web-like cracks.
Polyester resins cure to a crystal clear finish, which is desirable. They shall however yellow out as time goes, when they come into contact with UV light. They will also be malleable for a short timeframe, necessitating the artist to work fast. They are too hard to use after ten minutes. They also have a high and unpredictable shrinkage rate, which adds to the difficulty of handling them. It is normally used to bond fiberglass fabrics into hard sheets.
The polyurethane resins are more expensive of the two. They are however more versatile. They can be used in rubbers, plastics, inks, medicines, and more. They last longer and resist the effects of chemicals, oils, grease, and abrasion much better. They will also not emit the harmful toxic gases when casting.
They are normally presented as opaque and water clear resins. The costs they form will come with a lot of details and will not tear so easily. You can use it to make plenty of plastic options, from the hardest to the smoothest. They can easily form bubbles, which means you need to work on them in a pressure pot or vacuum chamber. You also need to have special colors to paint on them. This means that you have more limited color options for them.
The only major drawback with polyurethane resin is its moisture sensitivity. You need to work on it in the right environment. If you work in a humid environment, you should make use of a dehumidifier. It is therefore good that they cure faster. You thus need to move fast in your working.
When you consider these points, it becomes clear that polyurethane resins are the preferred choice for general mold making and casting works. They are the number one material in a clay studio. Polyester on the other hand is best for fiberglass fabrications.