Paint Stop and Overspray Filters

Core principle - Match the filter construction to the coating, spray load, booth airflow and downstream protection required. Install the full bank in the correct orientation, seal every edge, and base replacement on verified airflow, pressure trend and loading condition—not appearance or time alone.

An overspray arrestor is the dirty-air filter between the spray zone and the exhaust system. Its primary task is to capture paint droplets and solids before they deposit in the plenum, duct, fan or discharge path. It must perform this task without destabilising the designed booth airflow. A filter that captures well but is overloaded, incorrectly oriented, poorly supported or bypassed at the frame cannot provide reliable system control. [S171; S172; S175; S178]

This chapter addresses dry-filter liquid-coating booths. It does not treat ordinary intake filters as overspray arrestors, and it does not present particulate media as a control for solvent vapours or VOCs. Water-wash systems, powder-recovery systems and specialised emission controls require their own engineered arrangements.

Chapter objectives

After this chapter, the reader should be able to:

• distinguish paper labyrinth, expanded-paper, fibreglass, synthetic and composite overspray-filter constructions;
• explain inertial separation, surface loading and depth loading in practical terms;
• select a media family around droplet character, coating behaviour, spray load and downstream sensitivity;
• recognise when a single-stage or multi-stage system is appropriate;
• install media in the intended airflow direction with continuous sealing and support;
• interpret visible loading together with differential pressure and airflow condition;
• prevent unsafe mixing of incompatible coating residues;
• plan fire-safe filter handling and change-out;
• classify and route used media according to the coating residue and Malaysian waste requirements; and
• troubleshoot premature blocking, breakthrough, bypass, sagging and uneven loading.

Spray application produces droplets with a broad size distribution. Some deposit on the workpiece; the remainder becomes overspray. Larger wet droplets, fine mist, partly dried particles, fibres and coating solids can reach the exhaust face. The filter must retain the intended fraction of that load while allowing the ventilation system to carry air through the booth.

The following four functions should be evaluated together:

Capture efficiency and loading capacity are different properties. A fine stage may capture small particles but block rapidly if it receives the full coarse wet load. A high-capacity first stage may store substantial overspray but still require a downstream polishing stage where the exhaust system or environmental control demands finer capture. Selection therefore starts with the complete filtration duty, not one percentage.

Paper labyrinth filters use punched, pleated or folded paper layers to force the contaminated air through repeated changes in direction. Paint droplets have more inertia than the carrying air. As the air turns through the internal path, droplets tend to continue towards a wall or retention pocket, where they deposit outside the main stream.

The Andreae/AEREM catalogue describes this principle in a two-layer punched and pleated kraft-paper construction. Catalogue variants add paper capacity strips, synthetic backing or glass-fibre backing for different loading and final-capture duties. Treat these as construction families rather than interchangeable grades. Refer to the product label, Technical Data Sheet, or MKTECH representative. [S186; S187]

Paper labyrinth media is well suited to applications where wet droplets and useful holding volume favour inertial separation. Its performance depends strongly on correct expansion, orientation and sealing. Under-expansion or over-expansion changes the internal geometry; a reversed filter presents the wrong apertures to the spray side; and a poorly clipped edge creates a bypass path with almost no filtration.

Expanded-paper filters are made by slitting and stretching paper into a three-dimensional mesh or stack. Multiple layers may be offset to create a tortuous route with many impaction surfaces. These filters are generally compact, easy to cut and useful for moderate overspray duties, prefiltration or applications that benefit from a disposable paper construction.

The term “expanded paper” covers different layer counts, paper weights, slit patterns and optional backing media. Two products that look similar may have very different airflow resistance, wet strength, fire behaviour and paint-holding performance. Confirm the intended spray-side orientation and the declared application before installation.

Paper colour is not a universal grade code. Do not select a filter by colour, thickness or visual resemblance. Use the declared construction and application. Refer to the product label, Technical Data Sheet, or MKTECH representative.

Fibreglass paint-stop media consists of a resilient bed of glass fibres, commonly arranged from a more open spray side towards a denser clean-air side. Overspray collides with fibres and is retained within the depth of the media. The open upstream structure admits wet droplets; the denser downstream region provides progressively finer collection and support.

Fibreglass media is available as rolls or cut pads and can suit general refinishing and industrial liquid-coating duties. It must be handled without compressing the loft, tearing the backing or leaving exposed gaps. Compression reduces the usable depth and can increase resistance; stretching or sagging creates non-uniform flow.

Synthetic overspray media commonly uses polyester or another engineered fibre in pads, rolls, pockets or shaped high-surface-area elements. The fibre structure may be progressive, multi-ply, thermally bonded or supported by a scrim. Synthetic media can provide stable handling, consistent thickness and resistance to fibre breakage, but these general characteristics do not establish compatibility with every coating or booth.

Fibreglass and synthetic filters are depth-loading media: particles are collected through a volume rather than only on the front face. Their useful life depends on preserving that volume and presenting the intended coarse-to-fine direction to the airflow.

A multi-layer filter combines capture mechanisms or divides the load between separate stages. A coarse paper labyrinth, expanded-paper or deep-pocket stage can remove larger wet droplets and store bulk paint. A downstream fibrous stage then captures the finer mist that passes the first stage. Where required by the engineered exhaust system, a separate final stage protects downstream equipment or supports the specified discharge duty.

High-loading does not mean “leave the filter until airflow fails.” It means the construction is intended to store more paint at an acceptable resistance within its declared operating range. Uneven spraying, a blocked section, paint run-off, damaged media or a fan at its limit can require earlier replacement.

Use multi-stage filtration when one or more of the following conditions apply:

• the spray process produces both heavy wet droplets and fine mist;
• the first-stage face blocks before its internal capacity is used;
• downstream duct, fan or discharge cleanliness requires improved final capture;
• high production loading justifies greater storage volume;
• coating changes cause widely different overspray behaviour; or
• the booth supplier specifies staged protection for the exhaust system.

Adding a stage increases system resistance and may change airflow distribution. Confirm the fan duty, frame depth, seal arrangement and pressure-monitoring method before changing the filtration configuration.

Overspray behaviour is influenced by solids content, viscosity, atomisation, transfer efficiency, solvent or water content, cure chemistry, tack time, spray rate and distance to the filter. The same filter may load differently when any of these variables changes.

Compatibility is not proven by the carrier material alone. Check the complete filter construction, including paper treatment, fibre, binder, adhesive, backing, frame and any flame-retardant treatment. Check the coating Safety Data Sheet and the booth or filter supplier's declared application.

When changing from solvent-borne to water-reduced coating, or between resin systems, reassess overspray size, drying behaviour, residue compatibility, filter wet strength, pressure trend and waste classification. Refer to the product label, Technical Data Sheet, or MKTECH representative.

Use the following matrix to form a technically useful shortlist. It does not replace the booth design, coating data or product documentation.

Select by the normal production condition, not the clean-booth condition. Record the coating family, application equipment, transfer-efficiency condition, spray hours or output, exhaust-face area, current airflow, clean and loaded pressure trend, downstream cleanliness and waste route.

Air follows the lowest-resistance path. A small gap at a frame, joint or corner can pass a concentrated stream of unfiltered overspray while the media itself remains lightly loaded. The complete bank must therefore be treated as a sealed, supported airflow boundary.

Before spraying:

1. Confirm the filter identity and airflow arrow or declared spray side.
2. Isolate and secure the booth according to the site procedure before entering the filter area.
3. Inspect the frame, clips, grids, channels and seals; remove old residue by an approved non-sparking method.
4. Cut or expand the media without damaging its layers, backing or designed pleat geometry.
5. Install from one edge in a controlled sequence so every joint and perimeter remains closed.
6. Support rolls and pads against sag, flutter or collapse without crushing the media depth.
7. Check the upstream face for reverse installation, gaps, folds, tears or exposed frame openings.
8. Check the downstream side for bypass paths and loose media before restoring ventilation.
9. Verify gauge condition, booth airflow and containment before production release.

For Andreae-type accordion media, the manufacturer identifies correct expansion, white or declared spray-side orientation, secured first and last pleats and continuous top and bottom support as important installation conditions. Use the current instructions for the selected grade. [S186; S187]

No single observation proves filter condition. A dark or heavily coloured face may still have usable internal capacity; a clean-looking face may conceal deep loading, blocked pockets or a bypass path. Differential pressure shows resistance across the bank, but it does not directly prove airflow or capture.

Establish the clean-bank baseline after a correct installation. Trend pressure, airflow condition, fan state and loading pattern under comparable production conditions. The replacement point is the earliest applicable limit from the booth operating method, filter instructions, airflow verification, pressure trend, physical condition, fire control or downstream protection requirement.

Used overspray filters contain coating residue, not merely paper or fibre. The fire behaviour of the loaded filter depends on the coating ingredients, curing reaction, solvent content, deposited mass, airflow, heat sources and the filter construction. OSHA's spray-finishing rule provides transferable safety principles: control combustible deposits, keep mechanical ventilation operating during spraying and clearance, prevent ignition sources, protect dry-filter areas with the required fire-protection system, remove discarded media to a safe controlled location, and do not alternate filters between coating materials where the combination can promote spontaneous ignition. The applicable Malaysian requirements and the authority having jurisdiction govern the installation. [S175; S185]

Apply these practical controls:

• keep grinding, welding, hot work, smoking, unapproved electrical equipment and other ignition sources out of the spray and filter areas;
• maintain bonding, grounding, fan clearances, interlocks and fire-protection equipment as designed;
• do not use a filter with a coating known to self-heat or react unless the complete system is specifically approved for that duty;
• segregate residues from incompatible coating systems, oxidisers, organic peroxides or reactive two-component materials;
• do not stack warm, wet or actively curing filters in an enclosed combustible pile;
• keep upstream and downstream filter spaces accessible and free from loose deposits;
• use non-sparking cleaning tools and the approved residue-removal method; and
• stop spraying if containment, ventilation, an interlock, a fire-control system or the filter bank is not in serviceable condition.

“Flame-retardant”, “self-extinguishing” or a filter classification applies only under the stated test and product condition. It is not a guarantee that paint-loaded media cannot burn. Refer to the product label, Technical Data Sheet, or MKTECH representative.

Plan change-out for the complete bank or controlled zone so clean and loaded media are not mixed. Keep ventilation and isolation states under the booth procedure, and allow the required post-spray clearance before entry.

1. Stop spraying and complete the specified ventilation run-on and isolation sequence.
2. Confirm the coating identity, required PPE, handling controls and waste container.
3. Protect the clean side and prevent dry deposits from being shaken into the booth or plenum.
4. Remove media in manageable sections without cutting through heavily loaded pockets where avoidable.
5. Place used filters directly into the compatible, labelled container; do not leave them beside the booth.
6. Clean the frame and accessible deposits using the approved method without compressed-air blow-off.
7. Inspect support, seals, gauge connections, fire-protection components and the downstream plenum.
8. Install the replacement bank, verify direction and sealing, then establish the operating condition before spraying.

Do not wash, burn, compact or solvent-clean used filters unless the waste and fire-management system specifically permits that method.

The disposal route follows the captured coating residue and process, not the appearance of the clean filter. In Malaysia, the waste generator is responsible for classifying the waste from its source and materials. DOE lists contaminated paper or filters and paint-related sludge within the scheduled-waste framework where the relevant criteria apply; scheduled waste must be managed through the prescribed system and licensed facilities. [S188; S189]

Use the following sequence:

Do not assume that a paper or synthetic filter can enter general waste, landfill or incineration because the clean media may be combustible or marketed as disposable. The loaded filter must follow the classification and authorised route for its residue.

Use this sequence for a new filter, coating change or persistent performance problem:

1. Define the coating family, spray method, normal output and overspray behaviour.
2. Confirm the booth airflow route, filter-face area, frame depth, support and fan duty.
3. Choose the required capture mechanism: inertial paper, expanded paper, fibrous depth media or a combination.
4. Decide whether a high-capacity first stage and finer downstream stage are needed.
5. Check moisture, chemical, mechanical and fire compatibility for the complete media construction.
6. Confirm the intended direction, installation method, clean resistance and replacement criteria.
7. Establish the waste classification, segregation and container before the first loaded filter is removed.
8. Install and seal the complete bank; record the filter identity and date.
9. Verify airflow, pressure, containment and downstream cleanliness under the normal production arrangement.
10. Trend performance and revise the selection only through the controlled booth-change process.

• Never spray with missing, torn, reversed, collapsed or bypassed overspray filters.
• Keep the filter bank, plenum, duct and fan accessible and free from unsafe residue accumulation.
• Treat every used filter according to the captured coating, not the clean carrier material.
• Keep incompatible coating residues in separate controlled waste streams.
• Do not bypass filters or increase fan limits to hide a loaded or unsuitable bank.
• Keep ignition sources and hot work outside the controlled spray and filter areas.
• Maintain ventilation, interlocks, grounding and fire protection in their designed state.
• Use the coating SDS, current booth procedure and specified PPE during inspection and change-out.
• Stop work for smoke, heat, smouldering odour, sudden pressure change, overspray escape or downstream breakthrough.

The Filter Selection matrix appears on the following page of the printed handbook (page 276), outside this chapter extract.