Working With Polyester Resins

In this class we commonly use thermosetting (unsaturated) polyester resins for simple casting and fiberglass reinforced structures. This sheet provides basic use and safety information about those resins.

The polyester resins we use are a mixture of polyester resin, styrene monomer, an initiator, and a promoter. They are cured by adding methyl ethyl ketone peroxide which reacts with the promoter to create heat which along with the initiator causes an exothermic reaction that cross links the resin. The resins are available in several formulations. We commonly have both air inhibited and non-air inhibited resins on hand. Be sure you correctly select and know about the resin you plan to use.


Polyester resin gives off potentially toxic styrene monomer when it cures. All work with uncured polyester resins must be done in a well ventilated area, normally with an exhaust fan running. In our lab we also close the interior doors to reduce the amount of styrene monomer that follow air currents up the hallway. Read the MSDS sheet in the lab for additional information.

Eye and skin protection:

The MEK peroxide catalyst used is toxic and can quickly cause eye damage. Safety glasses must be worn whenever MEK peroxide is being used. It can also cause skin reactions and plastic gloves should normally be worn when working with MEK peroxide. Read the MSDS sheet in the lab for additional information.


MEK peroxide and styrene monomer are flammable and care must be taken to avoid any flame, sparks, or elevated temperatures that could cause a fire.


All materials must be stored in their original containers. For extended periods resins should be refrigerated to extend their shelf life.

Caution: Do not store resins and catalysts in areas used for food storage.

Use procedure:

To prepare polyester resin for casting or FRP lamination follow the steps listed below.

1. Cover the work area with newspaper and polyethylene sheet.

2. Read the directions on the material containers and review safety instructions. Note any instructions about mixing order for adding gel promoters or surface hardeners.

Note: The room temperature can effect resin curing, if the room is cold more catalyst may need to be added. Ask the instructor for assistance.

3. Assemble all materials including mixing container, stirring sticks, polyester resin, catalyst, mold release, molds, fiberglass and other laminating materials.

4. Cut and stage all lamination materials such as fiberglass and core materials.

5. Mix the catalyst with the resin following the instructions on the containers. The resins we usually use require approximately 6 drops of catalyst per ounce for castings and 8 to 12 ounces for thin laminations. If you are using gel promoters or surface hardeners carefully follow the mixing order on the containers.

Caution: Using too much catalyst will create stresses in the part and can actually start a fire.

6. Pour the resin into the coasting mold or saturate the laminate structure as necessary. Let the resin cure. Styrene monomer is given off during the curing cycle, maintain ventilation.

7. Clean up your work area and return materials to their storage areas. Hands and tools can be cleaned using solvents or special detergents. It is best however to avoid the need to clean tools and hands by using disposable tools and wearing disposable gloves. See the material containers and the instructor for more information.

Polyester HdOt 1997