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Spray finishing problems: causes and cures.

Implementing the correct solutions to industrial spray finishing problems begins with a basic understanding of their causes.

Creating a perfect finish requires a solid knowledge of surface preparation, finishes and spray painting equipment. But even an extensive knowledge of surface preparation techniques and paint chemistry is not enough to assure a professional finish. The finish must still be applied by a spray gun and all of the variables of its use must be mastered.

The equipment necessary to apply the finish -- spray gun, tank, cup, regulator, hoses, compressor, etc. -- must all be matched to the job as well as to each other. The equipment must be used and maintained properly, with an appreciation of how and why it works the way it does.

The moment of truth for any finish happens when the trigger is pulled. This article, excerpted from "The ABC's of Industrial Spray Finishing" published by DeVilbiss Industrial Coatings Equipment, focuses on that moment, and not only examines instances when something in the process goes awry, but more importantly offers insight into the possible cause of the problem and advice on how to correct it.

Before embarking on troubleshooting, though, an important reminder: Always prepare paint in a clean, dust-free environment. Paint has a remarkable ability to pick up dirt. Dirty paint will not only clog your spray gun, but it will also ruin your paint job. Get in the habit of pouring paint into the cup or tank through a paint strainer. Paint is never as clean as it looks.

Problem: Fluid leaking from packing nut

Cause/Cure:

1. Packing nut loose -- Tighten, do not bind needle

2. Packing worn or dry -- Replace or lubricate

Problem: Air leaking from the front of the gun

Cause/Cure:

1. Sticking air valve stem -- Lubricate

2. Foreign matter on air valve or seat -- Clean

3. Worn or damaged air valve or seat -- Replace

4. Broken air valve spring -- Replace

5. Bent valve stem -- Replace

6. The air valve gasket is damaged or missing -- Replace

Problem: Fluid leaking or dripping from front of pressure feed gun

Cause/Cure:

1. Packing nut too tight -- Adjust

2. Fluid tip or needle worn or damaged -- Replace tip and needle with lapped sets

3. Foreign matter in tip -- Clean

4. Fluid needle spring broken -- Replace

5. Wrong size needle or tip -- Replace

6. Dry packing -- Lubricate

7. Needle bound by misaligned spray-head (MBC guns) -- Tap sprayhead perimeter with a wooden mallet; retighten lock bolt

Problem: Jerky, fluttering spray Suction and pressure feed

Cause/Cure:

1. Material level too low -- Refill

2. Container tipped too far -- Hold more upright

3. Obstruction in fluid passage -- Backflush with solvent

4. Loose or broken fluid tube or fluid inlet nipple -- Tighten or replace

5. Loose or damaged fluid tip/seat -- Adjust or replace

6. Dry or loose fluid needle packing nut -- _ Lubricate or tighten

Suction feed only

Cause/Cure:

7. Material too heavy -- Thin or replace

8. Container tipped too far -- Hold more upright

9. Air vent clogged -- Clear vent passage

10. Loose, damaged or dirty lid -- Tighten, replace or clean coupling unit

11. Dry or loose fluid needle packing -- Lubricate or tighten packing nut

12. Fluid tube resting on cup bottom -- Tighten or shorten

13. Damaged gasket behind fluid tip -- Replace gasket

Problem: Top or bottom-heavy, or right or left-heavy spray patterns

Cause/Cure:

1. Horn holes are plugged -- Clean by reaming with non-metallic point

Note: Determine if the obstruction is on the air cap or the fluid tip. Do this by making a solid test spray pattern. Then rotate cap one-half turn and spray another pattern. If the defect is inverted, the obstruction is on the air cap and should be cleaned as previously instructed.

2. Obstruction on top or bottom of fluid tip -- Clean

Note: If the defect is not inverted, it is on the fluid tip. Check for a fine burr on the edge of the fluid tip. Remove with #600 wet or dry sandpaper.

3. Cap and/or tip seat dirty Clean

Note: Check for dried paint just inside the opening. Remove paint by washing with solvent.

Problem: Center-heavy spray pattern

Cause/Cure:

1. Fluid pressure too high for atomization air (pressure feed) Balance air and fluid pressure

2. Material flow exceeds air cap's capacity -- Thin or lower fluid flow

3. Spreader adjustment value set too low -- Adjust

4. Atomizing pressure too low -- Increase pressure

5. Material too thick -- Thin to proper consistency

Problem: Split-spray pattern

Cause/Cure:

1. Fluid adjusting knob turned in too far -- Back out counter clockwise to achieve a proper flow

2. Atomization air pressure too high -- Reduce at transformer on gun

3. Fluid pressure too low (pressure feed only) -- Increase fluid pressure (increases gun handling speed)

Problem: Starved spray pattern

Cause/Cure:

1. Inadequate material flow -- Back fluid adjusting screw out to first thread

2. Low atomization air pressure (suction feed) -- Increase air pressure and rebalance gun

Problem: Unable to form round spray pattern

Cause/Cure:

1. Fan adjustment stem not seating properly -- Clean or replace

Problem: Dry spray

Cause/Cure:

1. Air pressure too high -- Decrease air pressure

2. Material not properly reduced (suction feed) -- Reduce to proper consistency

3. Gun tip too far from worksurface -- Adjust to proper distance

4. Gun motion too fast -- Slow down

5. Gun out of adjustment -- Adjust

Problem: Excessive overspray

Cause/Cure:

1. Too much atomization air pressure -- Reduce pressure

2. Gun too far from worksurface -- Adjust to proper distance

3. Improper stroking (arcing, gun motion too fast) -- Move at moderate pace, parallel to worksurface

Problem: Excessive fog

Cause/Cure:

1. Too much, or too fast-drying thinner -- Remix properly

2. Too much atomization air pressure "Reduce pressure

Problem: Will not spray

Cause/Cure:

1. No pressure at gun -- Check air lines

2. Fluid pressure too low (with internal mix cap and pressure tank) -- Increase fluid pressure at tank

3. Fluid tip not open enough -- Open fluid adjusting screw

4. Fluid too heavy (suction feed) -- Reduce fluid or change to pressure feed

5. Internal mix cap used with suction feed -- Change to external air cap

Information for this article was excerpted from "The ABC's of Industrial Spray Finishing: A working guide to the selection and use of industrial spray finishing equipment," published by DeVilbiss Industrial Coating Equipment of Maumee, Ohio. To order a copy contact DeVilbiss at (419) 891-8200.
COPYRIGHT 1994 Vance Publishing Corp.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Article Details
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Title Annotation:Finishing Newsfront
Publication:Wood & Wood Products
Date:Jul 1, 1994
Words:1056
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