How to Properly Paint Wood Windows

Or: What I learned in 35 years of painting windows!

Tue, 29th Apr 2014
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I started painting sport fishing boats in the early 1980s. I learned early that it's all about keeping the water out. Whenever water can get behind a coat of paint it will begin to loosen the paint bond or worse, it will create a warm moist environment for wood rot to begin. Wood windows require a semi-annual inspection and attention to detail or rot will soon follow.

Older windows are usually single pane glass and require the glass to be held in place with glazing pins and glazing putty. Water collects in the thin gap that develops between the glazing putty and glass. Wood below the glazing putty expands when wet, and then shrinks when dry. The glazing putty is sure to fail under this condition. The proper repair of loose or missing glazing is covered later in this article. (Diagram C)

Worse yet is when water finds itself drawn into the gap between the glass and glazing putty and soaks into the joint between the stile (vertical board) and lower rail (horizontal board). When this joint expands and contracts it begins to loosen and deteriorate the joinery. This is the critical area to inspect, for it is here that most window and door paint fails and wood rot begins. (Diagram D)

When water finds a slender gap it is drawn into it and held there for days. Capillary action favors these thin gaps created by faulty glazing and also gaps that occur in loose joints commonly found at the stile and rail joint. Just open your window a few days after a rain. There will still be plenty of water sitting on top of the sill in full contact with the bottom of the window. (Diagram B)

Today's windows are typically dual glazed and held in place with a glazing tape and wood stop. All too often the glazing tape is slightly lower than the top of the stop creating a depressed area for water to collect. Water then soaks into the back side of the stop and into the stile and rail joint. (Diagram A)

Remove all soft wood until you reach solid clean wood. Then treat with a borate solution such as Board Defense. Let dry overnight. This insures that no new fungus will begin at the edge of your repair. You can see the remains of the pocket where a dowel was used to make the butt joint. This is one of the weaker types of joinery used in window construction. Worse yet, the dowels used were a soft wood and completely decayed. (See Existing Conditions image) (photo)

New dowel pins for repair of broken joint. I used 1/2" by 6" oak dowels and epoxy for a strong repair. A good tip is to cut a shallow kerf along the length of the dowel to allow excess epoxy to escape allowing the dowel to the bottom of the hole. This tip is not shown in the photo.) In this instance, I used System Three Rot Fix and Sculpwood epoxy products to repair the wooden window. Follow this link for complete product information and a short video on usage. 

Don't let the humorous 1950s styling of the video fool you about how excellent these products are. Make sure the Sculpwood is always placed into wet RotFix. There are two very important reasons for the blue tape. The first is to prevent any possible scratching of the glass when sanding. The second is to make sure that the primer and finish enamel fill any gap between the glazing and the glass.

After repairs we are ready for primer and two finish coats of enamel. One coat of primer and one coat of finish are not sufficient. (photo) I have two favorite coating systems - one waterborne from systemthree.com and a solvent based system from finepaintsofeurope.com. Both are far superior to what you would find in your typical local hardware or paint store. Fine Paints of Europe's primer is called "Oil Primer/Undercoat" and the finish coat is called "Hollandlac" and is made in gloss and satin. It is worth noting that any gloss coating will out perform the same product made in a lower sheen. Gloss coatings are also easier to clean.

System Three is a waterborne system. Their primer is called "Silver Tip Yacht Primer". It is a two component waterborne for marine use. Their finish coat is called WR-LPU. This topcoat is a two-part linear polyurethane coating available in colors only in a semi-gloss sheen. Gloss and satin are available in clear finishes.

Before the blue tape is removed inspect to see if there are any open gaps between the glass and glazing. Use a high quality clear caulking such as Dynaflex 230 to seal them. Lightly score the perimeter of the tape before removal. It sometimes helps to use a 6" drywall taping knife as a straight edge when doing this. (photo)

Step-by-step wood window painting process:

  1. Wash the windows to remove heavy soil or mold use a mild solution of TSP and bleach. TSP is available as a powder and premixed in many brands at most paint stores. Add 1 to 2 ounces of bleach to a quart of solution if there is any mold or mildew. Follow manufactures recommendations and wear gloves and eye protection.

    You will need to wait a week in average weather after doing this for the windows to completely dry. Trapping any moisture under your new paint will cause a blister in your new finish as moisture tries to escape. If it has rained recently wait 2-3 days prior to painting or use of a moisture meter is recommended. It takes longer that you would expect for the wood to dry completely.
  2. Outline the glass with 3M blue tape. I use the 2093El (green core and label) which is medium adhesion and good for 14 day removal. If you think you may work over several weekends choose the orange core and label 2080EL which is low adhesion and is good for 60 day clean removal. They are priced about the same. I like to leave a thin hair line (1/64th ") of glass showing between the tape and the glass. The reason is to make sure the paint makes a seal from the wood to the glass. The second reason is to make sure that after one primer and two finish coats that the tape is not trapped under the coatings.
  3. Sanding is the most important part of the job. If your home was built before 1978 you should check for lead paint. To learn more go to www.epa.gov/lead. If you want to check yourself go to www.leadcheck.com and buy an "Instant Lead Test" kit containing two swabs for about $9. Even with this precaution you may test negative on two windows only to find out that the windows on the other side of the home did contain lead.

    Best painting practices dictate that you should always work in a lead safe manner. Once you are comfortable with your decision I would use 120 or 150 grit for sanding surfaces in poor condition. 180 grit for windows in better condition. Either way you must de-gloss all surfaces to be painted. If you have a bond failure later the only way to fix it is to strip all your work and start over. Take your time and sand thoroughy. Cut a sheet into four equal pieces. Then fold each square in thirds. Make sure to sand carefully up to the tape but not on the tape. The tape will only afford modest protection. Clean with a dust brush or shop vac.
  4. Now is the time for any repairs. Follow the link above to SystemThree. I have found that repairs made with Bondo automotive type polyester type repair compounds do not last as long. They tend to loose their bond and soon water will get under them and rot will begin again. Bondo Home Solutions does now make a version specifically for wood.

    It certainly is better than spackle but I would go with the epoxy solution if you are able. If you must use spackle make sure to use an exterior rated brand. The technique for spackle is "prime-spackle-prime". Always sandwich the spackle between coats of primer.
  5. Priming begins after all repairs and a light cleaning to remove all dust created during your repairs. If you final color is anything other that white have your paint vendor tint the primer to approximate the finish color. It is best to prime 100% of the surface. If you find defects after priming, perform a spot repair and spot prime only the new repair.
  6. Before applying finish coats, do a very light sanding with 220 or 240 grit sandpaper. Be careful to not expose any bare wood. Caulk any open joints and let dry overnight. Be carful not to use a pure silicone caulking as paint will not adhere to it. There are many types of caulking, make sure the directions states it is paintable.

    If painting divided lite French windows paint the mullions first. (see diagram above.) Keep the stiles and rails clean. Next paint the stiles and rails. Last paint the tops and bottoms. I usually like to take the window sash out to paint the top and bottom. This also makes it easier to paint the jamb on a casement window. If you are not inclined to remove the sash from the jamb on casement windows buy a "Bender paint pad" made by Warner to slide under where you can't brush. It looks like of thin metal about the shape of a tongue depressor with a 1" by 2" piece of thin paint pad fabric glued on. The cost about $1.50.
  7. Lightly sand and clean again. Now you are ready for the final coat. The quality of your brush will play a major factor in your painting experience. Plan on spending 12 to 15 dollars for a quality brush. Purdy, Wooster and Corona are brush manufactures that I have used for years. There are many bristle types to pick from. Choose the proper bristle type based on the type of paint you are going to use. I like to use a 2 or 2.5" angled sash brush for trim work.

    For most homeowners after about 2-3 hours of painting you should clean your brush. Use mineral spirits (paint thinner) for the Oil base paints or water followed by a little bit of Krud Kutter for water based paints. Use a wire brush for final cleaning. Spin the brush to help dry it. Comb the bristles straight then put it back in the keeper and hang it up to dry overnight.
  8. The last step is to remove the tape and check for any gaps between the glass and glazing or wood stop. It is here that you will appreciate the straight lines and not having to scrape paint off the glass. If you ever need to scrape paint off glass, lubricate it with some soapy water or glass cleaner first. A fine bead of a high quality clear acrylic caulking such as Dynaflex 230 will insure a water tight finish.

    If your home has stucco walls make sure to caulk the transition between the stucco and wood as well.

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