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Old 06-27-2008, 07:54 AM
 
5 posts, read 18,567 times
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I’ve seen a lot of real estate agents over the years that had to deal with the front door or entryway of a home for sale. The cracked-and-peeling clear finish that has now failed…it’s one of the very first things a prospect sees when you show the house. A shabby-looking door can take a lot off the selling price, and it costs no more for the owner or a handyman to do it right, when refinishing is called for. Here’s an application note you can give them to turn that problem into a straightforward project.

The word “varnish” originally meant a coating made from the drying oils of certain nuts, berries or seeds. These oils react chemically with the oxygen in the air to form a relatively hard or flexible film, and have been used for protecting wood for probably thousands of years. Their “drying” or chemical curing is aided by small amounts of metal compounds called “driers” that may
be added during manufacturing. The oils are partly chemically reacted by the manufacturer as part of the production process, to make a more flexible, longer-lasting varnish coating on the wood.

This is necessary because wood expands and contracts with changes in the weather; temperature and humidity may cause more than 5% expansion or contraction of wood. An aged varnish film must stretch at least that much. These partly cured resins are very thick [viscous], and so some amount of solvent, usually mineral spirits, is added by the manufacturer to get a commercial product, easily applied, that will have good performance.
Other things a merchant may call a kind of ”varnish”, such as urethane varnish, water-reducible varnish, latex varnish…you may have to use these, if local rules require it. Some may be halfway decent for exterior use, most I don’t think are. This application note was written for Real Varnish. Many companies make real varnish, high-quality varnish. Marine supply stores sell good varnish, and in containers of quart or less, under the National Architectural coating regulation rules, solvent-limits don’t apply. One of the best is Epifanes, made in Holland but widely distributed in the U. S. You can find it easily with an Internet search, and order on-line. If you are using a real varnish, here’s an application note about how to apply it.
Dry film thickness determines life. That’s the first thing to learn. There are many reasons why varnish fails, and there are many ways it fails. A thicker film does not stop them all, but too thin a film guarantees prompt and certain failure. The wood will bleach and gray, the varnish will crack and peel, and that’s failure. So, we need some minimum film thickness. We will come back to this.

Varnishes also fail by loss of gloss. When you see this, it means it is time to apply another one to three coats. This assumes you start with enough of a film thickness. This recoating process is called maintenance. The ultraviolet absorbers in varnish protect the wood and primer underneath, but the chemical ultraviolet absorbers wear out. The new coats add more fresh ultraviolet absorbers, and restore the gloss of the finish.
For any varnish, a good rule of thumb is to budget a quart or liter for every twenty square feet [two square meters]. The most common reason for premature varnish failure is accidentally putting on too thin a total film thickness, by simply not knowing how much of a film thickness one has.

Budgeting a total usage gives some good hope that there will be an adequate minimum film thickness.

This is The Big Secret of how to get a varnish finish to last: Put on enough that it does not fail first by Ultraviolet Burnout of the wood underneath. The loss-of-gloss is the clue that you need to put on a few more coats.
I am not going to tell you how many coats to apply, because there is no standard definition for the wet film thickness of “a coat”. You and I hold our brushes differently, the weather is different here and there, we have different brushes and may even be using different varnishes. Some are typically 60% solids, some 70%, and the viscosities (the property of how easily a liquid flows is called viscosity) vary by about a factor of three from one varnish to another. So, your coat may be thicker or thinner than mine. The same wet coat thickness may sag on a vertical surface but not on a horizontal surface. Even on a horizontal surface, too thick a coat may gel and wrinkle. So, sometimes a solvent reducer may need to be added. Some people can apply varnish at a cooler time of day. Others may add some solvent reducer, others may hold the brush differently and apply more pressure to the bristle pack. You will do what works for you. The way to discover what that is, if you have not varnished much of anything before, is to practice on a few square feet of low-quality plywood. All the words here are no substitute for a little practical experience. Once you have read, and varnished, and read again, and varnished again, and have a certainty that you can do nice work, then start on your boat or front door or whatever the project is. There is a way to get a standard result. Budget a total amount of material to be used over a given area. Don’t sand heavily between coats, as that takes away film thickness that you need. Wet sanding with 220 to 320 grit just enough to take away the gloss and give a dull matte finish is all
you need. The next coat of varnish will chemically bond to the broken molecules of that sanded surface. Thin the varnish as needed so that it brushes out nicely, but you still must use that total amount of varnish over your budgeted area. Keep track of how much material you used in one coat that covered your area and handled nicely, and use that same amount the next time. On vertical surfaces you will be applying more coats, possibly thinned a bit more, so that you do not have drips or sags. Applying too much material on a vertical surface is what causes those drips or “curtain sags”, as they are sometimes known. Add a small amount of solvent reducer
(called reducer because it reduces the viscosity, making the material more runny) and the varnish will brush out in a thinner film, and then will not sag, but instead dry and harden as it should. On a horizontal surface it is easy to apply a very thick coat. Too thick is bad, for the varnish will not cure properly. It will skin over on the surface, stay gummy beneath, and the surface skin will eventually wrinkle and become ugly. Don’t let this happen. Varnish cures (dries) by a chemical reaction with the oxygen in the air. If the film is too thick, the oxygen cannot get to the bottom of the film before the top layer cures. Applying one fluid ounce (about 30milliliters) over 6-8 square feet will give a wet film thickness of 2-3 mils. Much thicker films may not dry and cure well.
There is no substitute for experiments. Reading about someone else’s opinion of what result you are going to achieve is not your result. It can help and guide, but if you have not done this before then experiment on a scrap of wood before you start the project. It is easy to get good results with varnish, but you must practice a bit first.

Solvent thinners do not affect cured varnish life or flexibility, but added oils can. Pay close attention to the manufacturer’s recommendations in this regard. The commonly available mineral spirits or turpentine can be used as a solvent reducer. In very hot weather these may evaporate too rapidly, giving a coat that does not brush out easily or allow wet edges to blend.

Some companies may make slower-evaporating varnish reducers that may be used instead of mineral spirits and will give better varnish flow and leveling in hot weather. Xylene is sold in many paint or home-maintenance stores, and is slower-evaporating than mineral spirits. That may be enough. Ask the people who distribute the varnishes, and the primers that you need to bond them to the wood.

The varnish-on-wood primers should be the penetrating epoxy type, that can dissolve the natural moisture in the wood. They soak in and thus can bond any topcoat to the wood. That’s the fundamental reason that clear finishes fail on wood; they were never stuck that well in the first place. That happened because of the old adage that oil and water don’t mix, and there’s a lot of truth in that. Wood is a water-type thing. Mineral spirits or turpentine is an oil-type thing, and so is varnish. I’ll tell you how that works:

Your paint- or hardware-store carries two types of clear finishes, and paints also. Those are waterborne and solvent-borne finishes, meaning whether is water or a solvent that carries the resins and then evaporates. Here’s the underlying applications principle: Wood naturally contains water. It’s part of the cellulose fibers that wood is made of. A typical varnish manufactured as a solvent-borne finish contains mineral spirits, a water-repellent solvent. The oilseed resins of that varnish are, basically, oil. The varnish resins need a petroleum/hydrocarbon solvent to dissolve THEM, and all those things won’t really stick to the cellulose fibers of wood, that already have water molecules on their surfaces. So, that’s why varnishes don’t stick the
best to bare wood. A water-borne finishes instantly hydrates the cellulose fibers of wood when it is applied, fully saturating them to their full water capacity, and now the suspended latex resin droplets are expected to stick to the damp wood fibers, and they don’t.

In the long run the finish peels because it was never stuck that well in the first place. So, that’s why a primer is needed, even you use a good-quality clear finish and apply a proper minimum total film thickness.

Well, there’s the basics. That should get you started, and through most jobs. Good luck.
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Old 06-27-2008, 08:52 AM
 
Location: Charleston, SC
5,615 posts, read 12,203,026 times
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Is a foam brush best, or a particular type of regular brush?
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Old 06-27-2008, 09:12 AM
 
28,056 posts, read 66,276,042 times
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I like a nice wood door. Most places these days have metal front doors. I have seen a lot of ugly ones, and they are not cheap. Once a metal door is dented I have heard that it can be repaired as though it were an autobody panel. Most people let it stay damaged. What a mistake -- makes the place seem worn down/damaged. Bad first impression!
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Old 06-27-2008, 10:08 AM
 
12 posts, read 112,624 times
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I've had to do a lot of minor maintenance work on my houses, and I've done what Pro Painter is talking about. It came out very well. Foam brushes usually don't stand up well to epoxy products, or those with solvents. I also don't like the way they snag on every little wood splinter. Natural bristle brushes are really bulletproof for applying varnish or the penetrating wood primers. The more expensive synthetic fiber brushes work, but cost more. The inexpensive natural bristle ones aren't worth the solvent to clean them, so I just buy enough for a job, and toss them when I'm done.

I like a nice wood front door, too. I think most people do.
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Old 06-27-2008, 11:22 AM
 
Location: Between a Rock and a Hard Place
110 posts, read 454,515 times
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Awesome post.

Now Buyers like myself can ask Sellers to varnish their ugly front doors after they get hardwoods and granite counter tops.

J/k it's Friday!
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Old 06-27-2008, 11:23 AM
 
Location: Charleston, SC
5,615 posts, read 12,203,026 times
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So bristle brushes, without applying too much material involved should prevent a lumpy finish? I've been looking into doing this with my front door, in addition to looking into strengthening the lead / solder that holds the glass panels together. A few joints are starting to crack a bit. Any advice on how to do that part? I'm great with soldering electronics but this is probably different.
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Old 06-29-2008, 11:08 AM
 
5 posts, read 18,567 times
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Quote:
Originally Posted by scuba steve View Post
So bristle brushes, without applying too much material involved should prevent a lumpy finish? I've been looking into doing this with my front door, in addition to looking into strengthening the lead / solder that holds the glass panels together. A few joints are starting to crack a bit. Any advice on how to do that part? I'm great with soldering electronics but this is probably different.
Lumps in the finish might be caused by applying varnish that contained
those lumps. Those can come from dried crumbs around the rim of the can that got
into the varnish and then ended up in a brushed-on coat. Those you get
rid of by filtering through a coarse or medium paint-strainer, or
cheesecloth. Lumps in a varnished finish might also be caused by
applying too thick a coat of varnish, so it did not level well. Not
enough reducer or a too-fast-evaporating reducer can cause that, as
well as working in too-hot weather, so the reducer solvent evaporates
too quickly, and the brush-marks don't have enough time to level.

Those cracks in the metal joints can be repaired with 63/37 tin/lead
solder and a soldering iron hot enough to melt a small puddle in the
metal. Usually the glass does not crack. Best procedure is to take the
door off, and lay it on a bench with some sheets of cardboard under
the glass section, just barely thick enough to reach and support the
glass but not push it up. Preheat the entire glass area with a hair
dryer. When you reflow the metal, the glass that is now supported from
below does not tend to fall. You will need to do it from both sides.
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Old 06-29-2008, 09:49 PM
 
Location: Charleston, SC
5,615 posts, read 12,203,026 times
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Thanks! Another thing to add to my to-do list. This also might help me out with a couple of pieces of stained glass I have. One is vintage, and has a couple of really weak joints while the other one is much newer but has a couple of broken pieces of beveled square / rectangular glass squares. Pretty sure I can scare up the glass somewhere but I never knew about the repair part.
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