Old varnish typically bubbles ("solvent pop" or blistering) due to trapped air, moisture, or underlying chemical reactions. As temperatures fluctuate, trapped air expands, lifting the softened or old finish. Other common causes include applying new varnish over uncured coats, poor application technique, or finishing in direct sunlight.
You will need to dilute the finish about 50% with mineral spirits or naptha, and as a result, you'll have to apply more coats. But its well worth it for the smooth finish you'll get in return.
The sealer coats, or first couple layers of varnish that are applied directly to the wood, may get bubbles as the varnish penetrates the wood, displacing air from it. Once the wood is thoroughly sealed, this should stop occurring.
To get rid of polyurethane bubbles, the best approach depends on whether the finish is still wet or already dried.
When you use vinegar on polyurethane finished wood, the acid in the vinegar breaks down the finish and starts to pit the finish. These pits trap sand and along with the acid will eventually strip the polyurethane.
Nobody wants to do all the work of refinishing and end up with bubbles in the finish. “The most common causes for bubbles are shaking the can and improper brush prep—you don't stick a brush into the finish without first soaking the bristles in mineral spirits, for oil-based polys, or in water, for water-based polys.
For thin layers of varnish, some homemade solutions can work: Baking soda + white vinegar: slightly abrasive action. Soda crystals diluted in hot water: helps soften the varnish film.
Cloudy or hazy polyurethane—known as "blushing"—is usually caused by moisture trapped beneath the finish, applying coats in high humidity, or using unmixed satin finishes.
If the primer/undercoat layer or existing varnish is still in good condition, it can be left on the surface. Sand with 280 grit and apply new fresh undercoat or varnish to achieve a good surface for the final finish. Single pack paint must only be repaired or overcoated with other single pack paints.
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.
To smooth out bumps in dried polyurethane, gently knock down the imperfections with 320 to 400-grit sandpaper or a sanding block, or burnish them with a brown paper bag. For severe bumps or air bubbles, carefully wet-sand the spot with 1000 to 3000-grit paper and apply a fresh, thin coat to blend.
Spray and brush varnishes each serve distinct purposes. Spray varnish is ideal for quick, even coverage on highly textured pieces, requiring no brush strokes but demanding well-ventilated workspaces. Brush varnish provides thicker, more controlled protection for large projects, though it requires meticulous application to avoid streaks.
To fix bubbles in wet varnish, immediately "tip off" the surface by lightly dragging a clean, dry brush over the finish at a 45-degree angle. If the varnish is already dry, lightly sand the bubbles down with 400-grit sandpaper, wipe away the dust, and apply a fresh, thin coat.
Remove varnish using a solvent gel
There are water-based gels such as Xanthan, Pemulen, and Methyl Cellulose, but also solvent gels. These organic solvent gels are prepared with Carbopol and Ethomeen.
The jar of petroleum jelly or Vaseline that you have in your medicine cabinet can work wonders to remove white water rings from furniture. The oily petroleum base penetrates the finish on the wood, displacing the moisture trapped inside, and the oils also nourish and condition dry wood.
If you've got these unwanted craters or bubbles, your best bet is to sand them away and then apply another layer of finish, but this time, go easy and be more careful by heeding to the following instructions: Depending on the surface you're working on, use 120-grit or finer sandpaper to scuff-sand the bubbles.
Yes, 3 coats of polyurethane are generally enough for most projects, as they provide a durable, protective film without making the finish look excessively thick or plastic-like. However, the exact number depends on the item's usage and the specific type of finish you are using.
Common Mistakes in Using a Polyurethane Sealant:
Murphy Oil is a vegetable oil based product. Most of our prefinished hardwood floors are polyurethane coated. Imagine polyurethane as the coating which protects your wood from stains, scratches and soil. You cannot oil this finish as the polyurethane is designed to not allow absorption of any material.
With the promise of food and protection, your porch or patio is irresistible to these little critters. While chemical-laden insecticides can effectively eliminate spider infestations, vinegar offers a non-toxic alternative. How does it work? Spiders dislike vinegar's pungent odor, and they avoid areas treated with it.
You can use a highly diluted mixture of vinegar and dish soap, but it should only be used as an occasional deep clean. Frequent use of acidic vinegar can dull and break down the protective finish on hardwood floors. Always ensure your mop is damp, not soaking wet, to prevent water damage.
Neither is universally better; the right choice depends on your project's shape, the polyurethane type, and the finish thickness you want.
These basic concepts can be applied to all of your paint and varnish projects, so please read them carefully. The traditional key to any great looking (and successful) varnish job is to build up numerous coats of the best varnish you can afford, sanding in between each coat until a flat and uniform surface appears.
The answer to "how many coats of polyurethane" isn't complicated. Follow the manufacturer's recommendations: 3 coats for oil-based, 3 coats for water-based brush application, or 4 coats for water-based spray application.