The Finality of Finishing: PRESERVING YOUR PLACE IN HISTORY
Sealing a work of decoupage with polyurethane protects it from scratches, dust, and oxidation. It can also give your work a high-sheen finish and, if you use several coats, even lend depth to the finished piece. But sealing is not always necessary; in fact, many times you will want to leave a work unsealed because a sealant would unwantedly alter the look of the piece. The finish of any sealer--be it matte, semigloss, or high gloss--will bring its own sheen, as will the amber tones of varnishes and shellacs. In addition, absorbent papers can soak up sealant and, as a result, actually change color right before your eyes. You might be lucky and really like the new color, or it might make you sick.
When deciding whether to seal a piece, first consider its end use. Is your tray intended to bear the burden of Cousin Ida's gin and tonics or is it merely going to hang on the wall? If it's going to get used, it needs to get sealed; serving trays, footstools, furniture, floors, refrigerators, cabinets, and other such things should be sealed.
Next, consider the aesthetic quality of your work of art, because, as noted, any sealer will import a specific look. If you've got a cachepot that's just going to sit on the mantle and you like the natural finish of the handmade papers you used, why muck it up with polyurethane? On the other hand, you might have antique on the brain, and a coat or two of amber-colored shellac would instantly age the piece. Be sure to experiment, since different sealants provide different sheens and color characteristics. In general, oil-based products ten to turn golden and mellow with multiple coats and age (unlike your spouse). Acrylic sealants dry clear and are not expected to darken over time. I say "expected" because they haven't been on the market for a real long time and I'm from Missouri, the "Show Me" state.
Old Decoupage was sealant happy. In some cases, thirty to forty coats were recommended for one small project. I've met some frustrated decoupeurs who are still trying to do this--in tiny work spaces, windows sealed. The fumes from all that sealant could account for their obsession with so many coats, just a case of old-fashioned brain fry.
Even for those eternally committed to protecting their decoupage creation, thirty coats is downright loony, twenty coats is overdoing it, and even ten coats will get you bumped from my dinner invitation list. You want to protect your work from scratches--not Hurricane Andrew, cosmic radiation, or the
Ebola virus.
In my opinion, a sealant overdose leaves your creation heavy and ugly. In my studio the only time we apply more than two coats of an acrylic-based polyurethane is when we have numerous layers of paper, over which coats of sealant will help you level the surface, or if we expect the piece to be very heavily used. If you decide to apply a third coat, sand the second coat lightly with a Grade 400 sandpaper. Then wipe the sanded surface with a water-dampened sponge or tack cloth before applying the third coat.
My thinking is this: If two or three coats of polyurethane will protect a floor in a new house (and that is the number many manufacturers recommend), you don't need many more to protect a floral tray; I don't care who you're inviting to tea.
For the purpose of
New Decoupage, I use acrylic- and oil-based polyurethanes almost exclusively. I rarely use shellac or varnish as they're yellow to begin with and the color only deepens with age. They are also messy if the humidity is high and can take forever to dry. And forget lacquer--this is the new age and this is
New Decoupage.
Polyurethane is available in two liquid forms: oil- and acrylic-based. Each is available in a formulation to provide a gloss, semi-gloss, or matte finish. Whatever you do, don't mix an oil-based polyurethane with an acrylic-based polyurethane. It's like mixing oil and water. Stick with one kind, at least until the finish is dry.
Acrylic-based polyurethane always dries clear and hard, even though it looks milky in the can and when first applied. As I mentioned, too many coats of an oil-based polyurethane imparts a mellow, golden tinge to finished work. Some people like the look, but I usually don't.
If I decide on a high-gloss, protective finish, my rule of thumb is this: Begin with two coats of acrylic polyurethane. Us additional coats whenever multiple layers of paper will benefit from the leveling effect of several coats. Finish with one or two coats of high-gloss oil-based polyurethane. The shine is better and the finish is beautifully smooth. Two coats, even three, will not cause serious yellowing.
One of the neat things about polyurethane is that the sheen of the last coat you apply is the effect you get. So why not start out with a high-gloss polyurethane for the first coat. See if you like it. If you don't, use semi-gloss for the second coat. When it dries, you'll have a semi-gloss finish. If that's not your facy, go right on to a coat of the matte finish; it's the last coat that counts. Because brands vary, follow the manufacturer's recommendation about drying time.