Clear finishes to protect wood or other materials can be polyurethane varnish, other varnishs, or latex finishes. I'm limiting this topic to interior finishes for cabinets, furniture, and the like, but not floors. Lacquers usually need to be sprayed on and I am not discussing them.
Characteristics
Polyurethane varnishes have been around for a
while and they give a hard surface. They dry with an amber color and turn even more
so as they cure, and some will give a shadow effect, darkening more around objects
placed on them as they cure. I prefer the convenience of the fast dry products, and
they seem to work as well as the others in most situations.
There are other varnishes such as marine spar
varnish that can be used inside for bar tops and such. They are tougher than poly
varnish and usually give a thicker coating; they also have an amber color like
poly varnish and they take longer to dry.
There are several types of latex finishes but they
all behave roughly the same. They usually dry clear and hard and stay that way.
The coating is thinner and more coats are needed to give adequate protection, but
they dry fast, allowing several coats to be applied in one day. Since they give a
thinner coat, brush marks are hardly noticeable. I have used both Varathane's Diamond
Finish and Minwax' Polycrylic. Be aware that Varathane now markets two varieties
of Diamond Finish; latex and oil-based. I have no experience with this new oil-based
product, and I see no reason to use it.
Preparation
As you would expect, the surface must be clean.
Wax, silicone furniture polishes, Murphy's Oil Soap, grease, and dirt will prevent
adhesion; these must be removed with the appropriate cleaner. Previous finishes that
are flaking off will have to be sanded or the flaking will show.
The surface should be sanded lightly with a fine
sandpaper (150 to 220 grit) to dull the gloss and provide some tooth for the new
finish. Then wipe down with liquid deglosser to remove the dust. Note that in preparation
for the latex finishes, this wipe down can be with a water damp cloth. A tack cloth
used to be the preferred method of removing dust but some prepared tack cloths can leave
a film that may prevent adhesion, particularly with the latex finishes, so avoid them.
Be sure you follow the directions regarding prep for the finish you are using.
Dust in the air will be a problem, particularly
with the slower drying finishes. If you have been sanding in the room, vacuum up the dust
and let the remaining dust settle before starting the finishing.
Removing An Old Finish
Sometimes sanding will take off the old finish
well enough. Power sanding with a palm sander can be a lot of work on a large job,
but probably not any more work than using a paint stripper. To get in recesses, you
can fold sandpaper and use the folded edge. Or wrap the sandpaper around a suitably
sized dowel or block.
You can use a paint stripper to remove an old
finish, but the stripper cannot be expected to remove the color of a stain since the
stain will have penetrated into the wood. However, you may do ok in removing a factory
finish since most of them are a colored lacquer that is mostly on the surface. The
results will depend upon the hardness of the wood to some degree; you will just have
to try it to see what happens.
In a couple cases I have had kitchen cabinets
stripped professionally and it lightened the color enough to be satsfactory. Professional
stripping usually involves on-site stripping of the cabinets and dipping doors/drawers
in a stripper vat. If you want to strip the finish, seriously consider a professional
job. It is not cheap but they are much faster and more thorough than you could be. The
chemicals they use are very toxic; I don't see how they can stand to work near a stripper
vat all day! By having it done professionally, your exposure to the chemicals will be
minimal. Don't even consider a sand blast strip job; it will just about ruin any piece
of wood.
If you want to strip something yourself, consider
using CitriStrip. This stripper (from a home center) works well, although it may not be
as strong as the commercial strippers. The beauty of it is that it smells nice! Be sure
to use the companion neutralizer wash to complete the job.
After using a stripper, or having an item stripped
professionally, you may have to stain some areas to equalize the color. On one job I had
to use lacquer thinner and a small brass brush to get the rest of the color out of the
wood in places.
Once the item is stripped, be sure the stripper is
neutralized and dry before proceeding. You will probably have to sand the items extensively
because the stripper may raise the grain.
Staining
There are many kinds of stains and I don't want to get
into that here. When cleaning up some cabinets, you may find it necessary to spot stain
to color light areas. The typical oil stain is easy to use for this. Apply it with a rag
or paper towel and rub off the excess. Do let it dry at least overnight before covering
with a clear coat, particularly a latex clear coat.
Where To Use
Poly varnish and latex finishes will work well
on kitchen and bath cabinets and furniture and similar items. In using them, take into
account the amber color of poly varnish and the clear or lack of color of the latex
finishes. For the latex, you may find it necessary to stain the wood if it lacks color.
The only way to decide what you need to do is to try the finish. I have used Diamond
Finish on several kitchen cabinet refinishing jobs with excellent results. A typical
situation would be where the finish is starting to show water damage but needs only slight
stain touch-up to be ready for the clear top coat.
Sheen
Most clear finishes are available in satin, semi-gloss
and gloss sheens. Expect the semi-gloss and gloss sheens to dull slightly as they cure.
The higher the gloss, the more durable; I wouldn't use a satin sheen in a high wear area.
The satin sheen, and to some degree the semi-gloss, contain a very fine silica as the
flatting agent. When using them, you must stir the product frequently to keep it from
settling.
Here is a tip for furniture quality work: The flatting
agent makes the satin and semi-gloss sheens a bit softer. Further, as you add more coats,
it gives a cloudy or muddy appearance that you may not want. The better way to do this
is to use one or more coats of gloss sheen and then the final coat of the satin or
semi-gloss. This gives you a solid base for the finish and greater clarity, and the
duller sheen that you want. Of course, you can also use rubbing compound to produce
satin sheen from gloss, but this takes time and the results may not come out uniform.
Application
Follow the directions on the can. Generally, two
or more coats of poly varnish should be applied, and fairly close together. Waiting too
long between coats may impair adhesion of the second coat and require sanding. For the
latex finishes, three or more coats are recommended.
An experience I had with Diamond Finish illustrates
that well. I was refinishing kitchen cabinets in a home and ran out of time after the
second coat. I couldn't get back there for a week. When I went back, the finish looked
very thin. I had to sand and put on two additional coats to get the film thick enough to
be satisfactory.
I recommend using a good synthetic bristle brush
for clear finishes, in general. I tried a foam brush with Diamond Finish once and was
not happy with the smoothness of the finish. Generally the manufacturer will tell you
on the can if a roller or paint pad can be used, and I would expect the answer to be No.
The first coat of these finishes may raise the grain
slightly in the case of the latex finishes and may collect dust some on the others. Expect
to have to sand between coats, paricularly the first and second coats. Use a fine sandpaper,
like 150 or 220 grit. Sand lightly, perferably with a sanding block. Power sanding is
not recommended; it will take off most of the finish. For a particularly smooth final
finish, use a Scotchbright abbrasive pad; you can get them in various grades at some stores.
Cabinets
When refinishing kitchen cabinets, use the technique
I outline in the painting topic. Repeating that here: Remove the doors and drawers and
number them with Roman numerals. Use a 1/4" chisel or straight-blade screwdriver and
hammer to mark them on the unseen edge; top of the top doors and bottom of the bottom
doors. Then make a sketch of the cabinets and copy the numbers to the sketch. This saves
time in putting the doors/drawers back in place when you are through. Then I lay a sheet of
plastic in the garage and lay the doors over the plastic on wood blocks. I keep a five
gallon bucket of 2x2x2 blocks for this purpose. You can finish the doors one side at a time
(back first) with minimum runs; wipe the edges with a rag or paper towel to prevent runs
there. Keep a log of the time you finish each coat so you don't lose track! This is
very important when you are applying three coats to both sides of doors.
Finishing Oak
Oak, ash and some varieties of mahogany have an open
grain. If you try to fill that grain with the finish, it will take many coats and a lot
of heavy sanding. Oak and ash in particular respond well to a paste wood filler. This is
finely ground stone suspended in linseed oil to make an almost putty-like filler. Follow
the directions on the can. Some call for thinning it slightly with paint thinner to make
it workable, and you may have to color the material to match the color of the wood.
Following directions you will basically work the filler unto the pores and wipe it off
the surface. Let it dry, then stain the wood and apply the clear finish. In this you are
getting into some sophisticated furniture finishing that is beyond the scope of this topic.
You can get books on the subject at libraries or home centers.
Pickled Finishes
A pickled finish or white-wash finish is seen most
commonly on oak, and will also work on ash. Basically, you paint on a white or off-white
color and wipe most of it off. This leaves the color in the pores of the wood and a thin
flash coat on the surface. You may be able to do this on wood previously clear coated; you
will just have to try to see if you like it. No matter what, you will have to sand off
most of the previous finish for starters.
You can get paints designed especially for this work.
I tried two of them once and found the latex totally unsatisfactory because it dried so fast
I couldn't wipe it off. The oil type worked fine. In one case I was trying to match an
existing finish and the prepared oil pickling was not the right color. I ended up using
some leftover off-white enamel I had in my stock, and that did the job. I had to
thin it a little to make it workable.
Once you have the color on and dried, then coat with
one of the clear finishes as above. Poly varnish will add an amber tone to the whitewash.
Latex clear coats will also to a minor degree. To minimize that with the latex whitewash,
let the whitewash cure for a week or more before proceeding.
Oil Finishes
An oil finish is usually either linseed oil or tung
oil that is wiped on without further protection. I do not recommend these. They never
harden and thus provide little protection to the surface. And they may water spot badly. Here
is the old-timer's way of applying an oil finish: Wipe on the oil, wait a few minutes
and wipe off the excess. Repeat this once an hour for a day, once a day for a week, once
a week for a month, once a month for a year and once a year forever. Does that give you
a clue? But see the next section for a better idea.
Wiping Finishes
The following is excerpted from an article in the
September 2003 issue of Wood Magazine, and this applies mostly to finishing new wood.
There are two types of these: oil/varnish blend and wiping varnish, and they may not be
labeled well on the can, until you read the directions. They all wipe on instead of
brushing. The oil/varnish types can contain either tung oil or linseed oil. These may
be labeled "finish" rather than varnish. "Danish oil" is usually this type, and it may
have color added. A wiping varnish is varnish thinned so it can be wiped on.
Raw linseed oil is an old time finish, but it is not
recommended since it takes long to dry (days or weeks); it remains soft and offers little
protection. "Boiled" linseed oil is the same thing with driers added; it dries faster
but is no more protection. Raw tung oil offers some water resistance and it dries lighter,
but it is still not a satisfactory finish.
A wiping finish may give the natural look you
want. Oil/varnish finishes may look great but offer little protection from scratching
and moisture. Wiping varnishes may also give that natural look but will give a harder
finish. Either type will require two or three coats, but more than that will not add more
protection. If you want a heavier build and greater durability, use the typical brushing
varnish.