Gel coat spray booths and composite booths: what to plan for

October 19, 2026 · Marathon Finishing Systems

A gel coat spray booth is an enclosed or open face booth that moves overspray and styrene vapor away from the sprayer and out of the building. It is not a paint booth with a bigger fan. The material is tacky, it gives off styrene, and the parts are usually the biggest thing in your building.

If you mold tanks, panels, hulls, decks or tooling in production, settle a few things before anything gets drawn. Here is what we go over at Marathon Finishing Systems, a spray booth manufacturer in Temecula, California, when a molder calls about our gel coat and composite spray booths.

Why is gel coat overspray a different problem?

Catalyzed gel coat overspray stays tacky and then cures hard on whatever it landed on: filters, walls, floor, light lenses. Paint overspray dries. Gel coat overspray loads a filter bank faster than most paint does, and it burns. That is your fire load.

Plan the cleanup before the booth. You will change filters often, so you want easy access and standard size media your supplier keeps on the shelf. Walls should be smooth and easy to clean. Booth coat is an optional add-on that lets overspray come off the coating instead of the panel. Catalyzed waste goes in a proper container and out of the building on a schedule, not in a pile in the corner.

If your air permit calls for a documented filter efficiency, tell us at the start. Higher-efficiency and NESHAP-rated exhaust filter packages are available when the permit calls for them.

Does a booth filter remove styrene?

No. Dry filters catch overspray droplets, and the vapor goes right through. Gel coat and polyester resin give off styrene, and a booth does one thing about it: it moves the vapor away from the person spraying and out of the building. That is what the airflow is for.

So the booth does not replace your respirator program, and a clean smell at the booth door tells you nothing about the sprayer's breathing zone. Ask your safety provider what your crew needs for the products you spray, and have the air sampled. Treating styrene in the exhaust is a separate system, and it is outside what we build. Your air district and an emissions-control vendor can advise on that.

Open face gel coat booth or enclosed?

Open face is the pick for access, and enclosed is the pick for heat and a clean finish. Open face gives full access. Air is drawn across the open front, past the sprayer and through the filter bank at the rear. For big molds, hull sections and parts you want to reach from every side, that access is hard to beat. But open face does not take heat, and shop dust can reach a fresh gel coat. If your building already holds the temperature your resin likes, and your shop is clean, it is a strong fit.

Enclosed booths have doors and filtered intake air, which keeps shop dust out of the finish. Dust in fresh gel coat means sanding and rework, so for some molders that alone pays for the enclosure. Reverse Flow and Semi Downdraft (we call it Modified Down Draft), Side or Full Down Draft booths can be built with heat. Front Flow is enclosed but does not take heat. Semi Downdraft is not a true down draft. It is essentially a crossdraft with the intake in the ceiling, so do not let anyone sell it to you as a down draft. A make-up air unit can pressurize a Reverse Flow or down draft booth, which keeps it balanced and helps keep shop dust out.

How big should a gel coat spray booth be?

Size it to your largest mold and the way it gets in and out, not to a catalog row. Is it on a cart, a lift, a rail or a forklift? Do you need to walk all the way around it?

Height, length and door size get set by the biggest piece plus the way you move it, with room around it for the sprayer and the film. Our drafting team draws the booth around your part, your cart or lift, and your building. Send us the dimensions of the largest mold and a photo of the space, and note that final dimensions can change once the design is done.

Product openings for long parts

Some parts are longer than any booth you would want to build. A tank section, a long panel or a continuous rail can pass through a product opening instead. Every opening is air that has to be pulled, so a big opening means a bigger fan and more make-up air. Bigger openings also weaken control of the air.

The answer is to keep the opening as small as the part and carrier allow, and keep the spraying well inside the booth. Our sales team works out the opening with you from your longest, tallest and widest part plus the carrier, so get us those numbers early. Each opening is quoted case by case. Flaps and curtains at an opening need a material your fire official accepts.

Catalyst: keep MEKP out of the spray zone

MEKP is an organic peroxide. It needs careful storage and careful mixing. Plan a separate, ventilated mixing area away from the spray zone, and keep catalyst storage to what NFPA 400 and your fire official allow. NFPA 33 also has sections for organic peroxide and styrene composite spraying, and your fire official will tell you which edition and rules apply where you are.

Our Paint Mixing Rooms are built for paint. We have not specified them for catalyst storage, so confirm any room with our sales team and your fire official before anything gets drawn.

Temperature, humidity and gel time

Gel time moves with temperature and humidity. Too cold and the gel coat sits wet and slow. Too hot or too dry and it kicks before you finish the pass. Get your resin maker's working range, then tell us. For an enclosed booth we can look at heat and at humidity control. A large booth moves a lot of building air, so make-up air matters, and your seasons matter. Say so up front.

EPA rules: materials, methods and records

The federal rules for boat building (40 CFR 63 Subpart VVVV) and reinforced plastic composites (Subpart WWWW) focus on what is in your resin and gel coat and how you apply it. They do not read like a booth spec. A booth helps with overspray and worker exposure, but compliance comes from your materials, methods and records. Many small shops fall under their air district rules instead of the federal ones, and those vary. Know what applies to you before you buy, ask your air district, and tell us what the answer is.

We do not handle permitting, air permits or fire suppression. Those stay with the owner and their contractors. Engineering documentation for the booth is available upon request to support your permit, and our sales team will size the booth and filters to the specs you give us.

What to bring to the first call

Send us the largest part size, what you spray, how parts move, your building and your climate, and what your air district has told you. We will tell you which booth fits and why. What is the biggest mold on your floor right now, and how does it get into the booth? Hop on a call and we will work it through.