What Size Compressor Do I Need For a GFRC Hopper Gun?

Article courtesy of Smooth-On, Inc.

What size compressor do I need for a GFRC hopper?

Whether setting up a new shop, or just trying to take on a weekend project, there are a lot of questions to answer about what tools you will need. This is a circular question in most cases, with answers that sound like "if this, then you do this- but if that, then you need to do something else". The first step of the process is to lay out the big picture of what you are trying to accomplish. What kind of work are you trying to take on? What kind of finishes do you want to create? What requirements come with those finishes? What are the space and budget limitations? And so on... The more completely you devise your set of questions and answers, the more efficient you will be in the process of creating your pieces. Needless to say, the needs of a DIYer, a professional specializing in custom work, and the facility focusing on production work are going to be very different. What is an absolute necessity of one situation would be foolish in another.

One question that will come up in virtually every shop is what size compressor you will need. Compressors come with some very useful specifications that will be necessary when making your decision. The most useful of these specifications is the available CFM (cubic feet per minute) at a certain PSI (pounds per square inch). Most tools will tell you exactly what CFM is required to run the tool. Many of the compressors at home improvement stores have a nice diagram that shows you which tools a compressor can effectively run.

One of the grey areas for artisan shops that are creating GFRC is how much CFM is required to run a hopper.This depends on variables that are widely considered personal preference. How much pressure an artisan uses in the gun, and the pace an artisan approaches a project with, make a steadfast rule about the minimum CFM necessary almost impossible.

All this said, what's the bottom line? How small a compressor can I use? I have sprayed mix with a compressor that provides 5.1 CFM at 90 PSI. What it meant for me is that I had to slow my pace on occasion to let the compressor catch up. It was functional, but I am accustomed to larger compressors, so it was a bit of a nuisance. I could see how someone that didn't know any different would be able to work with that size compressor without much issue, adjusting their habits to their tools.

In the final analysis, I would say you would be limited, but could get by with a small compressor (5-6 CFM at 90 PSI) if your budget limits your ability to go any other route. Before that decision is made, take a look on Craigslist and see what is available. You may find that you can pick up a much nicer compressor, for a similar price to a small compressor, if you are willing to do some leg work. Having a compressor that is capable of running a wet polisher, or a sander, will make your life easier in the long run. I would say it is worth a few hundred bucks to make the leap.


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.