Unlike lacquer and shellac, both popular wood finishes, varnish cures and won't soften when you moisten it with a solvent. You therefore need a chemical stripper to remove the varnish from furniture or woodwork prior to refinishing. When it comes to strippers, their effectiveness and agreeableness have an inverse relationship -- so you may have to split the difference between working quickly and working safely. You should first make sure the finish really is varnish, though.
Testing the Finish
If the finish you're trying to remove isn't varnish, you may be able to strip it with a solvent and sidestep the trouble of using caustic chemicals. You can often recognize varnish by looking at the finish -- it tends to be heavy, is often amber in color and you may see brush strokes, which are usually absent in lighter finishes. If you aren't sure, rub a small amount of denatured alcohol on the finish; if that softens it, the finish is shellac. Use lacquer thinner in the same way to test for lacquer. If the finish resists both solvents, it's polyurethane or alkyd varnish.
Methylene Chloride Stripper
You'll seldom have to resort to the strongest stripper, which is lye-based and caustic, to remove varnish, but heavy coats of polyurethane or spar varnish require a strong chemical. The toughest strippers for varnish, which work by dissolving the bond between the varnish and the wood, contain methylene chloride, usually mixed with other chemicals. Beside being a strong skin irritant and dangerous to your eyes, methylene chloride is a respiratory hazard -- it reduces your body's ability to carry oxygen and can cause cancer and kidney damage. Consequently, it's imperative that you wear goggles, gloves and a respirator while using it.
Other Solvent Strippers
Because of the dangers associated with methylene chloride, some manufacturers leave it out of their products, substituting N-methyl-2-pyrrolidone, which is less toxic. NMP isn't nontoxic, though. Its health effects aren't completely understood, and the trade-off for the substitution is reduced effectiveness of the product. Still other manufacturers market concoctions consisting of solvents such as toluene, methyl ethyl ketone and methanol. These strippers cling well to vertical surfaces and are effective for removing varnish, but they evaporate quickly. Among their disadvantages: they're flammable and too much exposure can give you brain damage.
The safest strippers for removing varnish are biochemical products derived from plants. Some of the chemicals in these types of strippers include citric acid from oranges and lemons; terpenes from pine; lactic acids from corn; and dimethyl sulfoxide from wood pulp. Some of them also contain NMP, but in reduced quantities. These strippers are generally safe to use, and they don't produce odors -- but they can take up to 24 hours to work. For this reason, these strippers, usually labeled "eco," aren't popular with refinishing professionals, but they are the best choice when safety is your first concern.