First, clean the surface carefully with mineral spirits or, for lacquer or varnish, denatured alcohol. If the entire finish is worn, clean the whole piece of furniture; you must remove all dirt and grease. Then apply a new coat of the finish already on the wood.
Varnish finish
Some varnish finishes can be reconditioned with a mixture of 3 parts boiled linseed oil and 1 part turpentine. Mix this solution well and apply it sparingly to the varnish with a cloth or grade 000 steel wool. This may work - it's worth a try.
A: The solution is to tint the new finish with dye. Varnish yellows with age and that's the reason your newly finished drawer doesn't match the old. I recommend using a dye called Orange Shellac from Wood Finishing Supplies. Don't let the name fool you; it's just a color and has nothing to do with real orange shellac.
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.
The patch will always be visible. You'd need to sand down the damaged portion and feather in your sanding on the edges to the good varnish, The grain may then need filling and even after that there's no guarantee that the color tone of the varnish will match.
To remedy the defect, the varnish must be allowed to harden thoroughly before flatting down and recoating. Prevent curtaining by applying the coating evenly and avoid heavy coats. Plan the work accordingly so that the wet edge is kept open.
Marine Spar Varnish can be recoated in 10-12 hours at the recommended coverage rate. The varnish can be recoated without sanding up to 24 hours. Beyond 24 hours, sanding is required.
Whatever product you want to use, as a general rule, you will have to completely remove any existing varnish, waxes, oils, wood stains, dust, dirt, grease, uneven areas and sticky stuff before applying a wood varnish.
Gloss oil-based varnish, polyurethane and Danish oil can last 10 or 20 years, though satin finishes and stains may fail sooner as pigments and flattening agents disable the driers. Water-based coatings and paints can also be viable longer than three years. Shellac, though, can go bad in under a year.
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.
A: Usually when varnish remains persistently sticky it's the result of application in a humid or cold environment. Sticky varnish can also be caused by too-thick application, or by re-coating an insufficiently dry layer. Traditional varnishes made in-studio (e.g. damar and mastic) are most prone to stickiness.
Apply a few drops of denatured alcohol to the furniture, as shown in the photo above. Wait a few seconds; then touch the spot with a soft-bristle brush or a cloth. Shellac, a popular finish before about 1920, will soften and turn a bit sticky. If it doesn't, it's not shellac, so move on to the next test.
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. This is the result of many centuries of experience and knowledge gained through trial and error.
If your existing urethane finish needs extra protection but not a total refinishing, you can re-coat it with Minwax® Helmsman® Spar Urethane, provided you follow the three rules for re-coating: clean, dry and scuff.
If your varnish isn't flaking and is in good condition, you can paint over the top of it. This is particularly useful if you just want to change the colour of the varnish.
Never more than 3 coats. First coat should be thinned 50% with mineral spirits for good penetration then two full strength, sanding between coats. When applying wipe on varnishes, double the coats. Penetrating oil finishes are so low in viscosity use as many as you like.
Instead of sanding, use a liquid deglosser to dull the existing finish. This product helps you prepare the surface for new stain or paint without the need to strip or sand. Wipe it on with a cloth, and it removes the glossy top layer, making the surface ready for refinishing.
Old varnish and stain can be removed from wood. Sanding is one of the most frequent techniques because it cuts through varnish and stain to reveal a new layer of wood. Although sanding is quick, it also generates a lot of fine-particle dust that is easy to breathe in, and it may remove more wood than you wanted it to.
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.
Medium Grit (80-120 Grit)
Medium grit sandpaper is the most commonly used type. It's perfect for general sanding to prepare surfaces for painting or varnishing. This grit range is typically used after the coarse grit in a multi-step sanding process.
Next layer applied too soon
When a new coat of varnish is applied to a surface that appears dry and is not fully dry, different minute sections of the surface interacts, and expands, differently to the solvents in the new varnish. Sand down, and depending on your visual requirement, repaint.
This method requires caution.
A heat gun can soften the varnish, making it easier to scrape off. Hold the heat gun a few inches from the surface and move it back and forth. When the varnish starts to bubble, use a scraper to remove it.