A varnish coating will have great difficulty adhering to the substrate (new or previously painted, interior or exterior) if any surface contaminants such as dirt, dust, oil, grease, mould, mildew, algae, salt, or other loose or powdery substance is present on the surface.
Sticky varnish occurs due to improper drying, excess humidity, or thick application. Fix it by: 1. Wiping with a mild solvent like mineral spirits or denatured alcohol. 2. Lightly sanding with fine-grit sandpaper (320-400) and reapplying a thin, even coat. 3.
It happens when the finish is not cured enough before you begin sanding. Let the finish dry a couple of more days. When applying many coats of an oil based finish, subsequent coat interfere with the oxygen getting to the lower coats of varnish. They cure much more slowly.
Varnish should be applied with a slow, deliberate stroke. Grip the brush like a pen and draw it across the wood, laying down a uniform coat. If needed, you can use two or three extra sweeps of the brush to distribute the varnish. Finish by drawing the brush from the wet area to the dry along the grain.
If you put the varnish on too thickly it might stay tacky indefinitely, because the solvent that's in it can't all evaporate.
Most of the time, residual stickiness will resolve itself but depending on thickness and condition of the coating, it may take an impractically long period of time. (Just maintaining a dust-free surface for more than a few hours can be challenging.)
Polyurethane finishes typically dry within hours and cure in 3–30 days. Drying and curing issues may be due to the following: Using old finish: Using an old or partially used can of finish can lead to issues with proper drying and curing. The finish may have thickened over time or become contaminated.
You can simply wipe on thinned varnish with a rag or you can even spray it. But when I want all the benefits that varnish has to offer, I apply it the tried-andtrue way — with a brush. Several coats of brushed varnish will produce a smooth, classic look along with all the protection a truly firstrate project deserves.
For a very durable finish and one that needs to be very tough, say on a kitchen table, coffee table or end table etc, 2 to 3 coats of varnish should be enough on the top, with 1 to 2 coats on the legs/base. For chairs, benches, chests and other such pieces, 1 to 2 coats should do the trick.
If you do decide to go ahead and apply new varnish over the top of old varnish, make sure that the existing varnish is scratch-free and do give the old varnish a light sanding using 120-grit sandpaper and wiping away any dust. This will ensure that the new varnish has something to bond to.
Your sticky problem is more than likely caused by the finish itself failing. Drying oils in the varnishes eventually destroy themselves and the sticky surface you describe is the result. Lacquers can suffer a problem caused by plasticizers migrating through the resins. One way or another, your finish surface is done.
Because of the durable nature of varnish and the fact that there is no pure solvent for all varnishes, it is difficult to restore a varnish finish. However, not all varnishes are alike. Some varnish finishes can be reconditioned with a mixture of 3 parts boiled linseed oil and 1 part turpentine.
Sand it down to remove uneven varnish. Clean with mineral spirits and reapply varnish. Do not apply excessive amount; use a cotton cloth wiping with the grain direction if necessary. Better to apply thin coats than thick ones.
: somewhat sticky to the touch. tacky varnish. also : characterized by tack : adhesive.
Using a heat gun
A heat gun works similarly to a stripper, by softening and loosening the varnish to make it easier to remove. A heat gun won't scorch the wood but you will still need to scrape off the varnish or paint.
Too much varnish. The symptom of you applying too much varnish is that it runs. Surprise! When there's too much liquid it flows somewhere.
Sanding Tips: No sanding is needed if you overcoat within 48 hours. Don't sand unless the most recent coat has dried for 24 hours. Use 320 grit to prevent scratches; sandpaper finer than 320 grit will not allow proper adhesion. Remove residue before reapplying.
While polyurethane is water- or oil-based plastic resin, varnish is older and made from resins, oils, and solvents. Because of the higher ratio of solids, varnish is less susceptible to ultraviolet light damage. This protection makes varnish an excellent choice for projects such as outside decks and exterior furniture.
Sand the surface smooth and flat before applying the final coat. Do your best to eliminate all the dust you can for this last coat. If you have done everything else well, your finish will look great even with a little dust. If the remaining dust bothers you, buff it out later.
It's best to use a brush with natural rather than synthetic bristles for oil-based finishes, and synthetic brushes (often called 'nylon' brushes) for acrylic or water-based varnishes. But you can also use rollers or rags for both kinds of finish.
Note that most polyurethanes do not adhere well to polypropylene, polyethylene, silicone or surfaces coated with fluoropolymers like Teflon.
Urethane can be stored for a long time as long as its kept dry and sealed tightly. You will know that its been contaminated or moisture got in if, when you open it, it looks like cottage cheese.