Download SPRAY APPLICATION DEFECTS: CAUSES AND CURES A perfect

SPRAY APPLICATION DEFECTS: CAUSES AND CURES
A perfect finish “straight off the gun” is not possible. There will always be imperfections.
Of course, it is possible to remove imperfections by sanding the surface level and then
rubbing it with fine abrasives or rubbing compounds, but this procedure is very time
consuming and usually well beyond the scope of a job. So it’s important to achieve the
best results possible during application.
To get good results, it will help to become acquainted with the most common application
defects and learn how to avoid, or at least reduce, each. The six most common defects
are dust nibs, orange peel, dry spray, runs and sags, blushing and fish eye.
Dust Nibs
Dust is always a problem no matter what coating you’re applying, but dust is especially
difficult to deal with if you don’t have a quality spray booth.
Besides keeping your equipment and coating materials clean, and working in conditions
that are as clean as you can reasonably make them, you’ll achieve the most dust-free
results if you use a fast-drying finish. The faster a finish dries, the less time dust has to
settle and become embedded in the finish.
The fastest-drying finishes, and thus the ones that produce the best results, are those
that thin with lacquer thinner: nitrocellulose lacquer, CAB-acrylic lacquer and pre-or
post-catalyzed lacquer.
Conversion varnish, which is thinned with toluene, xylene or similar petroleum distillate,
and shellac, which is thinned with alcohol, also dry fairly fast, and water-based finishes
dry reasonably fast if the humidity is low. Oil-based varnish and polyurethane are the
slowest drying of the common finishes and therefore the worst for collecting dust.
Orange Peel
It’s not possible to completely avoid orange peel, but it is possible to come so close that
the defect isn’t noticeable.
Orange peel can occur with any finish and is usually caused by spraying too thick a
liquid with too little air pressure. When stated this way, the solution is obvious: Thin the
liquid or increase the air pressure. If you’re using a spray gun supplied by a turbine
rather than a compressor, you won’t be able to increase the air pressure. In this case,
thinning is the solution.
Another common cause of orange peel is holding the gun too far from the surface or
moving the gun so fast that you don’t deposit a fully wet coat. The best way to
determine the proper distance and speed is to watch what’s happening in a reflected
light. By positioning yourself so you can see a reflection on the surface you’re spraying,
you will be able to see when the finish isn’t going on properly, and you can make the
necessary adjustment.
Dry Spray
Dry spray occurs at the edges of a spray pattern when the finish is drying too fast. It
also occurs when the finish bounces off the surface and then settles back onto it after
both the finish and the overspray have begun to dry. Dry spray occurs only with
fast-drying finishes and is especially likely to happen when spraying the insides of
cabinets, boxes and drawers where there's more bounce back and turbulence.
The obvious cure for dry spray is to slow the drying of the finish by adding a retarder –
that is, a slower evaporating thinner. Since the finishes that dry the fastest thin with
lacquer thinner, the appropriate retarder is lacquer retarder. Lacquer retarder can also
be used with shellac if you want to slow its drying. Xylene can be added to conversion
varnish if you want to slow its drying.
Don’t add more retarder than necessary to achieve an even wet surface, however, or
you’ll extend the drying time so much that the finish will collect dust and may run or sag.
Runs and Sags
The obvious reason a finish runs or sags on a vertical surface is that too thick a layer of
finish has been applied. Gravity pulls some of the finish downwards before it thickens
enough to resist the pull.
The problem is least likely to occur with finishes that thin with lacquer thinner because
it’s composed of a number of solvents that evaporate at different rates. Some of the
solvents evaporate before the finish even gets to the surface, and others evaporate
quickly thereafter. Because of this, the finish thickens rapidly allowing you to apply quite
a heavy coat without runs or sags.
Conversion varnish and shellac also dry fast, but their thinners evaporate at an even
rate, so runs and sags are more likely than with lacquer. Varnish, polyurethane and
water-based finishes are the most difficult finishes to apply without runs or sags.
It’s often not possible to completely avoid runs and sags, so the trick is to see them as
they occur and brush them out – usually spreading or removing some of the thickness
as well. You can see runs and sags as they are developing by watching the finish in a
reflected light as you are applying it.
Blushing
Blushing is a milky-whiteness that develops in finishes just after application. It is caused
by moisture in the air condensing into the finish and kicking the finish out of solution.
The discoloration is most likely to occur, of course, in humid weather, and the finishes
most affected are those that thin with lacquer thinner or alcohol and dry quickly.
Conversion varnish rarely blushes, and oil-based varnish, polyurethane and waterborne
finishes don't blush (though waterborne finishes often do turn somewhat opaque when
applied thickly).
To avoid blushing in lacquer and shellac, add lacquer retarder to the finish before
applying it. The retarder slows the drying process so the moisture has time to evaporate
before the finish hardens. The downside of adding retarder is that it slows the overall
drying of the finish, which may slow your production and could lead to dust problems
and runs or sags. For this reason, don’t add more retarder than is needed to avoid the
blushing.
If blushing has already occurred, spray some retarder (as little as possible) onto the
surface. Or, if you are using nitrocellulose or CAB-acrylic lacquer, wait overnight and
see if the blushing comes out on its own, which often happens. If it doesn’t, you can
spray the retarder on the finish the next day.
To avoid blushing in conversion varnish, add xylene or a solvent recommended by the
manufacturer (adding lacquer retarder may cause a previous coat to blister or lift). If
blushing cures in conversion varnish, you’ll have to strip it and start over.
Fish Eye
Fish eye, which is also called cratering or crawling, is caused by a surface-tension, or
slickness, difference between the finish and oils that have gotten into the wood. The oil
that causes the most problem is silicone oil, which is found in many furniture polishes,
lubricants and skin-care products.
You are unlikely to experience fish eye when finishing new wood, but it is common
when refinishing old wood and occurs most often with lacquers, conversion varnish,
oil-based varnish and polyurethane. The easiest and most reliable way to avoid fish eye
is to add silicone oil, which is sold as fish-eye eliminator, to the finish. This lowers the
surface tension of the finish enough so it flows out over the oils in the wood. You can
also "seal in" the oils by spraying a shellac sealer coat.
You may have to experiment a little to find the right amount of fish-eye eliminator to add,
but you shouldn’t add much more than an eye dropper full to a quart of finish or the
finish might develop a haze. When mixing fish-eye eliminator into varnish or
polyurethane, you’ll find that the mixing goes easier if you thin the oil first in mineral
spirits or naphtha and then add it.
Bob Flexner is author of “Understanding Wood Finishing” and former editor of Finishing
& Restoration magazine. His brochure on furniture care is available for purchase to give
or sell to your clients.
Please contact Bob by e-mail: [email protected] <mailto:[email protected]>
if you are interested in receiving a sample copy of this brochure.
Blushing
Runs and Sags
Fish Eye
Dry Spray
Orange Peel
Dust Nibs
by Bob Flexner
PRESS RELEASE
The first question almost everyone asks when their cabinets are installed or their
furniture delivered is, “How should I care for it?” No one has a better understanding of
this issue than Bob Flexner, author of the best-selling book, Understanding Wood
Finishing, and former editor of Finishing & Restoration magazine.
Flexner details what to do in his newly released brochure, “How to Care for Your
Furniture.” The brochure is being marketed to woodworkers, cabinetmakers, furniture
restorers and others to sell or give to their customers.
The core of the brochure is an explanation of how to choose among the four types of
furniture-care products: clear polishes, emulsion polishes, silicone polishes and wax.
The basics of furniture care (protecting wood surfaces from bright light and abuse) are
explained, and the four common ingredients contained in furniture-care products are
described along with what these ingredients actually do.
Also included is an explanation of the special considerations that apply to antique
furniture.
On a lighter note, the six most common myths – polishes moisturize or replace natural
oils, wax builds up, etc. – are debunked and reinforced with some entertaining cartoon
drawings.
If you would like to receive a free copy of the brochure, contact Bob at
[email protected]. <mailto:[email protected]>
<mailto:[email protected]>
next <../../Articles/December03/Products.html>
Page 13
next <../../Articles/December03/Products.html>
Top