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 shall also yield the clearness of glass, and have a high gloss finish after proper sanding and buffing. You however need to know which one applies in a particular job. It is therefore important to find out more about their differences.
Polymer resins are not the most expensive options there are. There must be a catalyst or accelerator for it to work as needed. They however produce a toxic odor, which makes it mandatory for an artist to wear a protective gas mask when working with it. The studio must be properly ventilated too. You also need to add in some form of reinforcement, preferably fiberglass. You shall also find them developing 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. Since they dry fast, they need an artist to finish up quickly. They will be unusable after about ten minutes. They can be tricky to use due to their high and unpredictable shrinkage rate. 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 have found work in rubbers, plastics, inks, medicines, and more. They can take the abuse from chemicals, oils, grease, and abrasion better than others. They shall also not give off the toxic gases while casting.
You can get them in opaque and water clear resins. The casts they form shall have a lot of details and will not tear easily. They also make for more choices, when you get the hardest as well as the softest plastics. Since they can easily form bubbles, you need to work on them in a pressure pot or a 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 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. The good news is that they have a fast curing time. You only have to work fast when mixing and pouring it to get good results.
These are the things that make you realize that polyurethane resins are better for general mold making and casting works. They are thus preferred in a clay studio. Polyester works well for fiberglass fabrications.