Artists will need polyester and polyurethane resins when it is time for clay modeling and other forms of casting. These resins are good for such work due to their ability to cure and form a hard cast. They can also come off as glass, and shine through when they have been sanded and buffed. But one needs to choose which of these two to use. To make a better choice, you need to know how different these resins are.
Polymer resins are not the most expensive options there are. 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. But they then turn yellow as they age after being exposed to UV light. They will also be malleable for a short timeframe, necessitating the artist to work fast. When ten minutes are up, they shall become useless. 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 however make for more versatile bases. They are necessary in rubbers, plastics, inks, medicines, and more. They are also hard enough to resist the beating from chemicals, oils, grease, and abrasion. When casting, they will not emit any harmful toxic gases.
They are normally presented as opaque and water clear resins. The casts formulated will display plenty of detail, and will not tear easily. You can use it to make plenty of plastic options, from the hardest to the smoothest. They are susceptible to forming bubbles, which necessitates treatment in a pressure pot or vacuum chamber. They also respond only to special types of paint. You thus have more limited color options to work with.
The only issue with polyurethane resin is the moisture sensitivity it has. It needs a dry climate for proper curing. If you work in a humid place, you need to invest in a dehumidifier. They also cure fast, which is ideal. You thus need to move fast in your working.
These are the things that make you realize that polyurethane resins are better for general mold making and casting works. They shall make good use in a clay modeling studio. Polyester works well for fiberglass fabrications.