How To Add A Clear Finish To Wood

by Darwin Hall

shellac on burl wood

Paint over any properly prepared wood surface and you’ll hide, or at least mask, its grain.  Clear finishes do the exact opposite.

They emphasize the wood’s natural beauty and all of its imperfections.

That’s why any clear-finishing job has to start with the wood itself.

Before you get into the painstaking preparation that a natural wood finish demands, take a close look at the wood grain.  Wetting the surface slightly lets you see it better and gives you a good idea of what a perfectly transparent coating will do.

If you don’t like a wood’s color, you can bleach it or tint it with stain — but no amount of staining or sanding will alter a grains pattern, obliterate knots, or hide surface imperfections or damage.

If you like what you see, prepare for some hard work — but ultimately very satisfying work.  Completely bare the wood, then sand it perfectly smooth.  Some “open-grain” species, such as oak, walnut, or mahogany, require a filler.  You may also need to seal the surface before applying the final finish.

Removing Old Finishes

If you’re refinishing a major piece of furniture that needs finishes stripped off, the path of least resistance leads to a professional paint stripper.  Generally for a modest price, these specialists will dip your piece in a caustic solution and have it back to you in short order, saving you lots of time and a great deal of effort.

See also:  Review: DAP Plastic Wood Professional Wood Filler

If a professional is not available, if a project doesn’t warrant sending it to one, or if you are stripping an item such as base or window trim that cannot be removed, there are a potpourri of other paint removing options:

Using Paint remover.  Used for any finish, thick or thin, on wood or metal.  Available in liquid or paste.  Paste is best for vertical surfaces.

Provide ventilation with paint remover; apply it with a brush or spray bottle, remove it and the old finish with a scraper and steel wool.  Wash off water-soluble strippers.

Using an Electric paint softener, or a propane torch.  Both remove old paint from wood; electric softener usually used on flat surfaces such as siding.  Use a torch with caution.  Provide ventilation.  Disadvantage of both is the danger of scorching bare wood, and both methods are tedious.

production sandingApplying Abrasives (sandpaper and steel wool).  Remove thin finishes; not for sanding heavy accumulations of paint (a seemingly endless job).

By hand, work is tedious; with a power sander, the finish comes off fast.

Use a contour sander in a drill for curved surfaces.  Be careful with power sanders not to groove the wood or cut through veneer.

Scrapers.  Excellent for removing old, dry paint from wood or glass surfaces.  Keep your scraper blade sharp.  Maneuver it carefully to be sure you don’t dig into the wood as you work.

Before you strip finishes, check a little deeper.

Some finishes that appear to need refinishing may only need cleaning.  If you’re in doubt, wash the item with mineral spirits, then give it three thin coats of wax.

If a white haze appears after you apply the thinner, buff it with extra-fine (000) steel wool before waxing.

Sanding A Refinishing Project

Sanding is the most important phase of every refinishing project.  If you’re working with old furniture pieces, first make any necessary repairs.  Re-glue weak joints, glue and clamp loose veneer, patch holes, and repair drawers and guides.

The amount of sanding required depends on the condition of the surface.  If it’s really banged up, start out by belt sanding with a coarse- to medium-grit abrasive.  Be careful because a belt sander cuts quickly and the edges of the abrasive can groove the wood surface.

Dust away sawdust after every step, then wipe the surface with a tack cloth.  If the surface is in decent shape, skip this first step.

For the next cut, switch to an orbital sander outfitted with medium- to fine-grit abrasive.  Though sanding with the orbital sander should yield a fairly smooth surface, don’t stop yet.

Unless you plan to bleach or water-stain the wood, dampen the surface of the wood to raise the grain.  Use a very fine abrasive on a sanding block to finish.  Apply only enough pressure to take off the “tooth” of the grain.

oil wax finishesLater, as you’re applying the finish, sand lightly between coats with fine sandpaper, this roughens the surface enough so the next coat can adhere to the previous one.

For final smoothing, substitute very fine wool for sandpaper.  Even this can quickly cut into sealers and undercoat, so use light, even strokes in the direction of the grain.

Choosing Clear Finishes

There was a time when the choice among natural wood top coatings could provoke a hot debate.  Should you choose shellac, varnish, oil, or lacquer?  Each finish has its partisans and its problems.

Then synthetics — chiefly polyurethane, but also epoxies and other plastics — arrived on the scene and settled the issue for most amateur refinishers.

Formulated for easy brushing or spraying, polyurethane dries rapidly; needs no rubbing or polishing, as oils and lacquers; and makes a surface more resistant to water, scratching, alcohol, grease, and everyday wear.

The question with polyurethane is when not to use this versatile coating.  Why polyurethane a large wall, for example, when you can protect it with shellac for a fraction of the cost?

Synthetic finishes have a few other drawbacks, too.  Most build up a thick plastic film that may not enhance a fine old piece of furniture; for a mellow, antique look, apply oil or a quality varnish.  Although polyurethane makes an excellent floor coating, you can’t smoothly touch-up scuffs and wear marks in heavy traffic areas.

Using Polyurethane Varnish

polyurethane applicationWhen you apply polyurethane varnish to a surface, you’re actually sealing it in plastic — a plastic so tough that hardly anything can penetrate it.

In addition to this exceptional durability, polyurethane is easy to apply; fairly fast-drying; super-resistant to chemicals and water; and available in low-gloss, satin, and high-gloss finishes.

As with most other finishes, you can apply polyurethane over any surface after completing the preparation steps discussed in this section.  If you’ve applied a stain or wood filler to the surface, make sure the surface is absolutely dry before applying polyurethane.

Apply polyurethane with a brush, roller, or spray gun.  If you plan to spray it, you may have to thin the finish.  Be sure to check the thinning recommendations on the container before you start working with it.

All types of wood require at least two coats of polyurethane.  The first coat serves as a primer and sealer; the second serves as a finish coat.  Sanding is desirable between coats if dust or lint gets into the wet finish.

It also makes it easier to tell where you applied the following coat.  When you apply the finish — both coats — keep the work between you and a light.  In this way, you will see the missed spots as the finish is applied.

Missed spots are caused by poor penetration into the wood or inadequate application.  Missed spots leave little dimples in the finish, and they’re almost impossible to spot-in after the material has dried.

Not all polyurethanes are clear.  Some are colored to resemble pigmented shellac.  With these, you’ll usually need to apply several coats of the finish to reach the color that you want.  Each coat of finish will produce a deeper tone, so make a test application on scrap of the same material to determine the number of coats you’ll need.

If you reach the color tone before achieving the sheen you want, let the surface dry thoroughly, then apply clear polyurethane finish to complete the project.  The clear finish will not change the color tone underneath.

If you want to apply a clear polyurethane finish over colored paint, the first finish should be in perfect condition.  Don’t expect the polyurethane to hide any defects in the material — it won’t.

Low-gloss polyurethanes cost more than high-gloss types and are less durable.  Use low-gloss finishes for top coats to cut the shine off high-gloss coatings underneath.

As with other varnish and shellac finishes, dust and dirt control is critical with polyurethane.  Work in a still room with no puffs of hot or cold air from heating and cooling ducts.

Don’t do anything to cause dust to become airborne, especially sweeping the floor just before the finish is applied.  Instead, use a tack cloth to remove dust from the work.

Using Lacquer

Lacquer produces a very smooth, quality finish.  It dries super-fast, making it a dust dodger.  After it dries, you can rub away dust and brush marks from its surface.  It’s also inexpensive and available in clear finishes and a variety of colors.

Lacquer’s fast drying time also is one of its disadvantages.  It dries so quickly that you must correct mistakes immediately.  Another disadvantage is that you can’t apply lacquer over painted finishes.

clear wood finishesThe lacquer’s solvent will lift off the paint finish that lies underneath it.  Again, proper preparation is a key.  Prepare the surface as you would for any other clear wood finish.  Lacquer may sag and run.  It’s best to use many thin coats with a spray gun.

If you spot a sag or run, let it dry, then remove the defect using wet or dry sandpaper.  Or, wipe the run or sag with a soft, lint-free cloth saturated with lacquer thinner.  Spot-fill the area and continue on with the work.

The blemish will show after the lacquer has dried, but it won’t be noticeable after the surface has been rubbed properly with steel wool or rubbing compound and wax.

Generally, for lacquer to look and perform its best, you need to apply at least three coats.  Unlike most clear finishes, you don’t have to smooth the surface with sandpaper or steel wool between coats because the material “dissolves” and blends into preceding coats.

After the lacquer has dried for 48 hours, finish by rubbing the surface with very fine wool and hard wax or rubbing compound.  As you do this, work in a small area.  Completely rub out this area before you move on to other areas.  Otherwise, the compound will dry and be hard to buff out.

Using Oil Finishes

That deep, rich patina you see on old gun stocks and some antique furniture probably consists of nothing more than boiled linseed oil and turpentine — coat upon coat, laboriously rubbed into the wood’s grain.

You can do the same yourself.  Just combine two parts oil with one part turpentine, pour it on, rub off the excess, and let it dry completely.  Repeat and repeat and repeat until you’ve totally saturated the grain, a process that may take up to 10 applications and dozens of hours of tiresome rubbing.

Achieving this lustrous effect needn’t be that difficult, however.  Using a commercial resin-oil finish, sometimes referred to as Danish oil, will ease the work load considerably.  Like ordinary oil, these penetrate into the wood for a surface that’s more than skin deep.

They also contain a synthetic or natural resin that hardens inside the wood grain.  The result: a finish that actually toughens the wood, yet doesn’t call for nearly as much rubbing.

Usually, you need only two or three applications of resin oil to get a deep, lasting finish.  Unlike linseed oil, it dries overnight, doesn’t gum up in warm weather, and rarely needs to be renewed.

Yet, its just as resistant to stains, scratches, minor burns, water, and alcohol.  If damage does occur, you can just rub it out with sandpaper or steel wool and apply more penetrating oil.  Unlike polyurethane varnish, oil lends itself to spot repairs.

Penetrating resin oils vary somewhat.  Some include varnish, others plastics, and still others are combined with wax.  A few also come in different weights to suit open- or closed-pore woods.

Many come in various colors, or you can tint them for staining effects.  Read the manufacturer’s instructions before applying a resin oil.  Many go on with a few easy steps:

  • Flood the surface with oil and spread with a brush or cloth.  The first coat will soak in quickly.
  • Wait a few minutes, then test the surface.  If dry, apply more oil.  Wait a few minutes and wipe off the excess.
  • Sand lightly before the second coat to remove any raised grain.  After this, don’t sand again.
  • Apply subsequent coats with the grain, rubbing hard.  Let dry 24 hours between coats.  Apply more coats until you’re satisfied with the finish.
  • Apply the final coat with your palms; your hands supply heat.  If desired, finish with a hard wax.

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