Artists have the options of polyester and polyurethane resins when it is time to do clay modeling and 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 you need to deduce which one to use. You therefore need to know more about their characteristics.
Polyester resins are the cheaper alternatives in the market. It needs there to be a catalyst or an accelerator for it to work well. When you are casting, it shall give off a toxic odor, which means you need to have been prepared with a protective gas mask. The studio must be properly ventilated too. It then needs reinforcement in the form of a filler, such as fiberglass. After a while, they tend to develop spider web-like cracks.
Polyester resins will also cure to a crystal clear finish. There is however the danger of them yellowing out with time, when they are struck by UV light. They shall also be malleable for a short time, which needs the artist to work fast. They are too hard to use after ten minutes. They are sometimes difficult to use, due to their high and unpredictable shrinkage rate. IT makes for a good bond between fiberglass fabrics.
Polyurethane resins, on the other hand, are more costly. They however make for more versatile bases. They are necessary in rubbers, plastics, inks, medicines, and more. They last longer and resist the effects of chemicals, oils, grease, and abrasion much better. When casting, they will not emit any harmful toxic gases.
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. They also make for more choices, when you get the hardest as well as the softest plastics. They can easily form bubbles, which means you need to work on them in a pressure pot or vacuum chamber. You need to also invest in special paint to work on them. This makes its choice of colors more limited than other options.
This material is however highly sensitive to moisture. It needs a dry climate for proper curing. If you work in a humid environment, you should make use of a dehumidifier. They also cure fast, which is ideal. You only have to work fast when mixing and pouring it to get good results.
From this analysis, it is clear that polyurethane resins are the better choice for general mold making and casting works. They are thus preferred in a clay studio. Polyester on the other hand is best for fiberglass fabrications.