Since it's not considered a VOC and HAP, you can use as much as you need for cleaning or to make your finishes work
You probably don't use it very much, but you should be aware that acetone is becoming a more important solvent, primarily because it's the only commonly available solvent except water that isn't classified as a VOC or HAP.
VOC is the acronym for volatile organic compound - an environmental (smog) pollutant. HAP is the acronym for hazardous air pollutant - something that is bad for us to breathe. Though acetone has a strong odor making it seem toxic, it is actually a fairly benign solvent in vapor form, limited at low exposure to causing only mild irritation of the central nervous system.
Not being a VOC or HAP means there are no regulatory restrictions on how much of this solvent we, or the manufacturers we buy from, use. No other commonly available solvent, except water, is so free of restrictions.
Acetone is also very useful because it is miscible (mixable) with all common solvents and water and thus most common paints, finishes and coloring products. So acetone can be added in significant percentages to most of the coating materials we use.
Also important, acetone is the fastest evaporating and one of the strongest of all commonly available solvents and it is very dry (non-oily). So it makes an excellent cleaner and degreaser and this is how it is used in most other industries.
The fast evaporation corresponds, however, to high flammability. A flame or spark can set off an explosion or flash fire if vapors build up enough. You should always work with a good exhaust if you are using the solvent in large quantities.
The strength means that acetone can damage or remove most paints and finishes, so you should avoid using this solvent as a cleaner on all but the most solvent-resistant finishes. These would include conversion varnish, two-part polyurethane, UV-cured finish and epoxy resin.
VOCs and HAPs
Because acetone is an exempt solvent, finish manufacturers can include as much as they want in their products. This is very significant because it makes possible the continued availability of many products - especially lacquers and stains - that would otherwise be taken off the market in areas with strict environmental laws.
For example, lacquers require a great deal of solvent (often 75 percent or more) to be sprayable and NGR dye stains are entirely solvent. Many areas, including California, limit VOC content to as low as 27.5 percent, (275 grams/liter). So there would be no way, or at least no inexpensive way, for lacquers and dye stains to comply if it weren't for acetone.
While this is good for us because it keeps products we use on the market, it changes the application characteristics significantly. Any finishing product that contains a large amount of acetone dries very rapidly.
Fast drying can cause dry spray and blushing in finishes and toners. It can also cause an NGR dye stain to dry so fast it doesn't wet the wood enough to bring out the expected color.
As I've explained previously in this column, finishers in strict areas such as California overcome the often too-fast drying by adding some very slow evaporating butyl Cellosolve, which is widely available from distributors and paint stores that sell to the trade.
Doing this may take the finish out of compliance, but it's not illegal to sell or buy the solvent.
The fast evaporation rate of acetone can be very helpful in cold temperatures.
Because acetone is compatible or mixable with most finishes and stains, you can add the solvent to speed drying. Acetone is especially effective with shellac and every type of lacquer. Water-based finishes, however, tend to coagulate when acetone is added. You should test first.
Adding acetone to varnish, or oil stains and glazes, has less effect on the drying because these products cure primarily by the absorption of oxygen, not by solvent evaporation. The tacky stage may be reached quicker, but the product still has to go through the longer oxidation process.
Acetone is very commonly used as a cleaner and degreaser in labs and in industry. It is also used as the active ingredient in fingernail polish removers and as the solvent for removing epoxy and cyanoacrylate (CA) adhesives from hands or other surfaces before the adhesive hardens.
More specific to finishing, acetone is the best cleaner for removing the resinous oils in oily woods such as teak and rosewood - much more effective than naphtha or denatured alcohol. The naturally occurring oils in many exotic woods can weaken the bond of water-based adhesives and finishes and significantly slow the curing of oil and varnish finishes.
To improve the bonding or speed the drying, wipe the surface of the wood with acetone just before applying the glue or finish. Wait until the solvent evaporates, but don't allow so much time that the oils rise back to the surface of the wood.
Because acetone is miscible with water, it's also very useful for removing residue water from spray equipment when switching from a water-based to a solvent-based product.
And because acetone is miscible with mineral spirits, it's very useful for speeding the cleaning of varnish, oil-stain and oil-glaze brushes before washing in soap and water. It's the residue mineral spirits that makes washing so time-consuming because of the number of washings necessary to remove it.
The acetone removes the mineral spirits and then dries out quickly after spinning or shaking the brush.
Acetone's solvent strength makes it the most effective solvent for removing masking tape and stickers that have been stuck to a finish for so long they no long peel off. But the solvent might also damage or remove the finish, so it's usually best to try weaker denatured alcohol or naphtha first.
The solvent strength makes acetone excellent for removing paints and finishes, so it is a common ingredient in paint and varnish removers. Evaporation is retarded by the inclusion of paraffin wax, which rises to the surface of thickly applied remover and forms a barrier.
It's with the purpose of not disturbing this barrier and releasing some of the solvent that you are commonly instructed to brush in only one direction.
We need to reconsider our relationship with acetone. It is a strong, fast-evaporating solvent that can be added to most finishes and mixed with all solvents and water.
But what makes acetone really stand out as we become more environmentally conscious is its exemption as a VOC and HAP. We can use as much acetone as we need for cleaning or to make our finishes work.
Bob Flexner is the author of "Understanding Wood Finishing."
This article orginally appeared in the January 2010 issue.