Finishing problems & solutions: spray, curtain & roller coating

Photo By AcromaPro

Listed below are some common finishing problems related to spray finishing, curtain coating and roll coating applications. 

Spray finishing

Problem: Pin holing

Causes: Insufficient and excessively dry spraying of the substrate; Excessive application at high temperature or on heated substrate; Porous substrate (fiberboard, particleboard, etc.); Incorrect thinning

Solutions: Rub down the defective surface; Check viscosity of the coating; If necessary, use a slower thinner; Introduce other changes/measures to inhibit pin holing

Problem: Striping

Causes: Defective or dirty air or fluid nozzle; Incorrect gun settings; Conveyor speed and the guns transversal speed not synchronized

Solutions: Check and clean the nozzles; Check the spray gun positions and spray fan; Synchronize conveyor and transversal speed

Curtain coating

Problem: “Blue” corners

Causes: Edges too sharp; Viscosity too low; Corners sanded through when sanding primer

Solutions: Increase the radius of the corner either by denibber or in the woodworking process; Increase viscosity; Use a less coarse sandpaper when sanding the primer.

Problem: Gloss and tone variations

Causes: The jaws of the curtain coater are not parallel, applied amount uneven; The curtain is moving forward and backward when coating, which results in an uneven application amount

Solutions: Adjust the jaws so they are parallel; Shield the curtain from air movements

Problem: Sagging on edges

Causes: Too high conveyor speed; Coating – thinner not optimal

Solutions: Decrease the conveyor speed; Contact your material supplier

Roller coating

Problem: Ghost image of previous panel

Causes: Application roller does not re-wet to the same level as for the previous panel

Solutions: Apply scraper to the application roller

Problem: Line formation, crosswise

Causes: Different speed on conveyor and application roller; Damaged application roller; Too high down pressure on application roller; Pressure between doctor and application

roller too high; Doctor roller running reverse at too high of a speed; Not enough material between rollers when running doctor roller reverse; The bearings in either rollers or conveyor are damaged

Solutions: Check application roller and conveyor speed (rollers must be clean); Check for damage on the roller; Use less down pressure; Release pressure between rollers; Slow down reversing doctor roller; Increase the pump pressure; Change bearings

Problem: Line formation, lengthwise

Causes: Damaged scraper on doctor roller, or in case of a light filler machine the scraper of the smoothing roller; Damaged application roller; The protective shutter in the UV oven is set too low (infeed); Ghost image of previous panel

Solutions: Replace damaged equipment; Replace damaged equipment; Adjust height of shutter; Not enough coating on roller

Problem: Scrape marks and lip formation in front or end of panel

Causes: The speed of the application roller and conveyor belt is not synchronized; Smoothing roller running too fast or at too high pressure; Too much material applied before smoothing when filling

Solutions: Adjust the speed of the application roller; Decrease the speed of the smoothing roller and/or release pressure; Decrease application amount in light filler machine by pressing doctor and application roller together, or change speed of doctor roller

Source: AcromaPro, “Facts on Finishing” – The Sherwin Williams Co. For information call 888-277-1418 or visit


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