Orange peel is the most common
phenomenon encountered in finishing, and most frustrating. The dilemma however,
is that most finishers attempt to cure the symptoms and not the disease thus,
unable to remedy the problem nor prevent from occurring in the first place. An
orange peel is a waive type of finish whose textured surface resembles that of
an orange peel. This phenomenon generally occurs predominately in spray
application with fast and quick drying finishes, such as lacquers or quick
drying varnishes. That does not necessarily mean that orange peel cannot occur
while brushing on a finish or, with a slow drying finish, that however, tends
to be rarity.
There
are many causes or contributing factors to orange peel. The most pronounced
ones are: fluid viscosity of the finish; wrong type of thinner used;
incompatible thinner to the finish; product and ambient temperature; size of
the atomizing set in the spray gun; spray pressure; undersized compressor; and
surface contamination.
A
heavy viscosity material will not flow properly through most conventional spray
equipment. Its thickness cannot be atomized properly by the pressure and volume
of air delivered by the spray gun. Kicking up the pressure from say 40 to 60
psi will reduce the orange peel somewhat, but never enough for a perfect
finish. Reducing the viscosity suitable for spray application with proper
reducer (thinner) will generally solve the problem provided viscosity is the
key contributing variable. Using an improper thinner however, can contribute to
orange peel as discussed further below.
It is important to recognize that reducing down the
viscosity to improve the flow out reduces the solids as well. Consequently,
more coats are needed to build a desired finish. Equally important, lowering
the solids below 28% for the topcoat when spraying deems the lacquer
finish not VOC compliant under the Clean Air Act hence, subject to fines in
many major cities and some states such as New York City, its surrounding
counties and all of New Jersey.
There
are viscosity cups that are used to measure the flow of the finish. The two
most popular are the #4 Ford and #2 Zahn cup. The #2 Zahn has a smaller
discharge hole at the bottom of the cup. The cup is filled to the rim then the
time is checked in how long it takes for the cup to empty, up to the point when
the fluid drips. Therefore, a suitable viscosity would be 17 to 20, and 20 to
22 seconds through # 4 Ford and #2 Zahn
cups, respectively, at 68 degrees.
It
is important to mention that there are special solvents readily available known
as “flow-out” solvent that help reduce or eleviate orange peel. These solvents
are used sparingly, 1 to 2% of total finish, and are not designed nor used to
thin the viscosity, but will some just by their addition. These reduce the
surface tension of the finish hence the finish flows out better eliminating
orange peel. Most finish suppliers have such solvents available as an accessory
product.
A fast drying thinner used in hot weather will cause the surface film to dry extremely fast. The bottom film is still trying to flow while the top film has skinned over and no longer flows causing an orange peel affect. A lacquer thinner containing acetone as the first ingredient (predominant) will cause this phenomenon to occur (and blushing too) because of its extremely fast evaporation rate.
A finish spray applied onto a surface consists of multiple plates of film flowing in one direction - away from the spray gun. When the top film dries much faster than the bottom film an orange peel affect develops. That is because the bottom film is trying to flow while the top film has skinned over and can no longer flow. Therefore never use a fast drying type of thinner during hot and/or hot and humid weather. Become familiar with the ingredients of your thinner and how much of each in the blend so that you can adjust its drying time to suit your needs.You can add a small amount of retarder to slow down the drying time of the thinner, up to 10% by volume. Caution, overloading with a retarder such as Butyl Cellosolve, can leave a finish soft for as long as thirty (30) days. This issue is addressed in another publication.
INCOMPETIBLE THINNER TO THE FINISH
Not all thinners are compatible with all finishes where one or more of the ingredients within the thinner may be incompatible with the ingredients of the finish. For example, a lacquer thinner too rich in acetone will cause many of the pre-cat lacquers to orange peel because acetone can cause swelling of the resin(s) in the lacquer and/or make the lacquer dry too fast thus, skin over. With conversion varnish, for example, most require the use of xylene (xylol) as the proper thinner. However, many finishers use lacquer thinner instead. In few cases, the negative affect is minimal. In most however, orange peel, among other problems will occur, such as crazing or cracking months after the job is finished.
COLD TEMPERATURE
If the shop temperature and/or the finish is too cold, say below 50 degrees, viscosity will increase, causing orange peel. Warming up the product few minutes prior to spraying to decrease the viscosity is an excellent remedy. Place the cup with the finish (attached to the gun), pressure pot, or pail in warm water for several minutes. Do not place on hot surface or near an open flame if the finish is flammable or combustible; fire or explosion will occur. Mix a bit to equalize the temperature then spray. You should see a significant difference in flow out properties of your finish.
It is important to recognize that, even though you warm up the shop and you feel comfortable, and the product is warm, orange peel still occurs. Why? Because the surface temperature of the piece you are finishing is still cold, nearly at the temperature of the shop when you first came in. Because of its low heat transfer coefficient, it takes wood a long time to warm up; many hours from a cold start. As the bottom film of the sprayed finish lands on the cold surface viscosity of that film increases soon upon impact. The top film is warm and wants to flow while the bottom film is cold (more viscous) resisting flow causing orange peel.
ATOMIZING SET TOO SMALL
With the modern finishes where, they tend to be VOC compliant and ready to spray as supplied, an atomizing set (needle, tip and air cap) should be around 1.5 mm (0.059 inch) rather than 1.0 mm. The 1.0 mm atomizing set used to be the standard designed to spray lacquers that had to be thinned 50% with lacquer thinner and became nearly water-thin after thinning.
The modern lacquers, however, are thicker in viscosity as supplied versus a thinned out standard lacquer. Using an atomizing set below 1.4 mm to spray these types of lacquers, and current water-based finishes, will result in an orange peel from slight to severe as the size decreases, that is, at 0.8 mm the orange peel will be severe unless the finish is thinned with a suitable thinner to compensate for the small orifice.
SPRAYING PRESSURE TOO LOW
There are many of the newer HVLP conversion spray guns (work off a compressor) that specify 15-20 psi of inlet pressure. However, at this pressure orange peel will occur with most of the aforementioned modern lacquers, unless highly reduced in viscosity. These guns were designed for automotive finishes that have very low viscosity as supplied and lower resistance to flow than the modern lacquers. As such to spray lacquers through the types of guns without an orange peel will require around 40 psi inlet pressure. Increasing the pressure, however, increases overspray; it is impossible to get around the problem with the current technology.
Even with conventional spray guns, lowering the pressure below the recommended, say below 30 psi, will cause orange peel unless the lacquer is thinned to near water thin viscosity.
UNDERSIZED COMPRESSOR
The newer HVLP conversion guns that work off a compressor require a minimum of 10 cfm of air delivery to properly atomize the modern finishes. That converts to about 3 hp compressor requirement. The tank storage capacity in gallons should not be confused with the cfm air delivery, since these are mutually exclusive and cfm is just as critical to good film formation as above mentioned spray pressure. A 1 hp compressor will not lay down a nice finish because it can only deliver about 3 cfm to the HVLP conversion gun leaving it air starved and resulting in orange peel.
SURFACE CONTAMINATION
Surface contamination will cause orange peel because these contaminants impose a resistance to the flow of the bottom film of the finish. This phenomenon is typically found when refinishing furniture or kitchen cabinets where they have been cleaned, polished or waxed with products containing silicones.
To ascertain whether you will be subject to such contamination, after stripping off a finish or over an existing finish to be resurfaced, conduct the following water test. Take a cup with some water and a clean cotton cloth and pour some of that water onto the horizontal surface of the piece to be finish, say tabletop. If the water flows like a river you will most likely not experience orange peel or fish-eyes. On the other hand, if the water refuses to flow and pulls inward into a nearly perfect circle, you will experience orange peel, fish-eyes, pin holes, sweating or, combination of all three. Wipe off the water immediately upon conducting the test to avoid water spotting.
The problem with silicone contamination is that most silicones are insoluble in solvents or water. As such, cleaning a stripped surface with lacquer thinner or other solvents will not alleviate the problem. Emulsification, as with dish washing liquid works like a charm. Scrub the contaminated surface after sanding with our Fish-Eye Remover/Dewaxer or Murphy’s Oil Soap, at full strenght using a scrubbing brush or coarse scrubbing pad. Scrub with the grain then wipe with dampened rag with water. Let dry for few hours or if you cannot spare the time to wait, wipe thoroughly with denatured alcohol after the water dampened rag. You can sand again if need be. Stain and finish as normal. You should no longer experience orange peel. You can repeat the above water test gain, just to be certain.
Should you experience orange peel from surface contamination, let that coat dry thoroughly, overnight preferably. Wet sand thoroughly with our Fish-Eye Remover/Dewaxer or Murphy’s Oil Soap at full strength then wipe thoroughly off with a damp rag. Let dry for at least one hour. Your next coat should be orange peel free.
SPRAY DOUBLE PASS COAT
Unless your dealing with severe surface contamination or severly undersized spray equipment, you can prevent orange peel by spraying a double pass coat instead of spraying only in single direction. A double pass coat represent spraying with the grain, waiting 15 to 30 seconds then spraying against the grain. The two perpendicular films will tend flow into each other preventing orange peel. However, if the finish dries too fast, the orange peel will diminish, but not be eliminated completely. Therefore, give your finish ample drying time to level out properly.