What is the difference between Spray-Up and Premix?

Article courtesy of Smooth-On, Inc.

Spray-Up: Spray-Up GFRC refers to the process of using a spray pump, either rotor/stator or peristaltic, to spray a GFRC slurry and continuous strand AR glass fiber roving.  It uses a specialized spray gun with an integrated fiber chopper.  The GFRC slurry and fiber are fed to the gun separately and the chopper cuts the fiber and introduces it into the slurry stream at the spray nozzle.  Spray-Up typically contains a minimum of 4% glass fiber.  In the US and several international markets, Spray-Up GFRC “skins” are panelized on a steel stud frame and weigh 20-25 lbs per square foot.

Premix:  Premix GFRC incorporates AR glass fibers, or chopped strands, into the slurry during mixing and slurry preparation.  The Premix process is further broken down into various production techniques such as spray premix, cast premix, pultrusion and hand lay-up.  Premix is most often used for smaller GFRC parts and rarely incorporates a frame.  Without the frame, GFRC will weigh 7-10 lbs per square foot.


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.