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Fill Sill Weather Checks (final)

Posted: November 18th, 2011, 5:28 pm
by johnleeke
Fill Sill Weather Checks
Status: final treatment standard
Update: 6/5/13
Author: John Leeke
Contributors: Jay Treiger, Paul Marlowe, Richard Byrne, Jan Scopel
References: Save America’s Windows, p. 24, 2009 (illustrations courtesy John Leeke)

Title of Treatment: Fill Weather Checks
Class of Treatment: [ ] Maintain, [ ] Stabilize, [X] Repair, [ ] Upgrade, [ ] Exception
Type of Treatment: [ ] Traditional, [X] Contemporary

Condition to be Treated: Weather checks in the top weathering surface of wood sills. This method is also effective for weather checks in casing boards and other exterior woodwork.

Description: Deep cracks, called weather checks, often occur in sills on the south and west side of the building. The purpose of filling weather checks is to provide a continuous surface that is smooth enough to hold paint and to drain water down the slope of the sill. Minimize damage to the outer surface of the sill when removing paint, cleaning out the checks and trimming the cured filler. The checks must be filled completely down to their narrow bottoms. There must be bands of bare wood at the top surface between the checks that have no epoxy treatment at all so moisture can escape from within the sill. It is seldom necessary to make the surface perfectly flat and true. A flat true surface carried across the whole sill might look odd next to weather beaten exterior casing and clapboards. Usually the somewhat uneven eroded contours and “corduroy texture” of the sill can be accepted as a sign of age as long as the checks are filled and the surface can drain.

Typical Procedure:
1. Remove Paint from sill. Minimize damage to the wood. In some cases paint can be left in place when the paint film is in good condition. With the paint in place epoxy materials may be less likely to soak into the wood surface between the checks.
2. Clean out checks, removing all dust and debris down to the narrow bottom. Scrape inner sides of checks to expose bare bright wood. Minimize damage to the wood.
3. Dry the wood of the sill.
4. Prime checks with epoxy primer or consolidant. Apply with a narrow spouted bottle and artist's pallet knife to keep the primer in the checks and limit the spread of the primer onto the surface of the sill. Apply a few or several times as it soaks in to assure complete penetration.
5. Fill checks with epoxy paste filler and allow to cure.
6. Trim filler flush with surrounding wood. Minimize damage to the wood.
7. Sand surface of sill.

Long-term side-by-side comparative field testing has shown that epoxy fillers provide a longer-lasting repair than other types of fillers. Epoxy materials formulated especially for wood-epoxy repairs should be used. The epoxies adhere tenaciously to the sides of the checks, flex with seasonal wood movement and provide a good base for paint. Other materials could be used with this method, such as, traditional glazing putty made of linseed oil and whiting or contemporary sealants made of acrylic or other flexible compounds. These other materials may have a much shorter maintenance cycle than epoxies. But, they may provide other advantages, such as reversibility, which may needed in a conservation context.

· Epoxy liquid resins, 2 part, to use as a primer and consolidant, formulated especially for wood-epoxy repairs
· Epoxy paste filler , 2 part, made of epoxy resins and powdery fillers, formulated especially for wood-epoxy repairs
· Solvent for clean up
· Rags for clean up

Quality of Results:

Best Work: Strips of bare untreated wood are exposed between the filled weather checks, allowing moisture to migrate out of the sill. Epoxy filling is flush with surrounding wood and fills the entire check to the very bottom. The sill surface forms a contour that is flat enough to hold a paint system well and to drain well.

Inadequate Work: Epoxy forms a thin skim or thick plate that covers large sections of the surface of the sill, which can trap moisture leading to decay of the sill. Some weather checks are not filled.

Re: Fill Sill Weather Checks (draft)

Posted: April 19th, 2012, 11:21 pm
by jay
I often use epoxy to renovate sun checked cracked sills. I find that a liquid epoxy creates a deeper crack penetration and is undeterred by a bit of dirt or grit lodged deep in the crack. Liquid epoxies benefit from the admix of wood flour (fine sawdust) but not enough to stop the 'flow'. Usually several applications are necessary. I emphatically agree with John that the wood sill needs to 'reappear' and that the epoxy is not coating the surface Liquid epoxies dry much harder than wood and sanding and or grinding is both tedious ,messy and may easily damage the antique wood we are trying to save.prior to proper painting. Here is where a heat gun and a glazing knife (five in one) come to the fore. Cured epoxy will soften somewhat with heat, becoming easy to scrape or carve away. Wood, on the other hand is unaffected by heat allowing epoxy to be even safely removed from detailed woodwork. I have used this method for 10 years or so after a fortuitous experiment. I find that hardened epoxy is no longer difficult to remove

Re: Fill Sill Weather Checks (draft)

Posted: April 20th, 2012, 7:46 am
by johnleeke
Jay, thanks for your detailed comments.

Liquid epoxies...Usually several applications are necessary.

I agree, as I too usually apply the liquid epoxy a few or several times as it soaks in. This is a key part of the procedure, and I had left it out. So, I'll add it in to the Standard above, and add you as a contributor.

Cured epoxy will soften somewhat with heat, becoming easy to scrape or carve away.

I have never thought of this and will give it a try. This technique could be very product specific. What product are you using? Have you used this technique with more that one product?

I sometimes try to do the trimming (with a sharp chisel or scrapers) when the epoxy is about 90% cured and still somewhat soft, although this must be done with great care so the epoxy remaining in the checks is not disturbed.

These two techniques are just a couple of examples of the many subtle nuances of this work that each worker develops based on their own specific skills and knowledge. In this first round of developing the Standards we are focused on documenting the method and procedures, leaving the more specific techniques used up to each worker. In the end, best work results are dependent on the skill and knowledge of the worker, and not on the procedure or the products used.

I find that a liquid epoxy creates a deeper crack penetration and is undeterred by a bit of dirt or grit lodged deep in the crack.

This is also what we though when we started treating these sill weatherchecks with epoxies back in 1979. By the mid- and late-1980s some of our early repairs were failing and we had the chance to repair them, or in some cases replace the sills. We cut the old sills open to investigate the failures, and discovered that in most of the failure cases the liquid epoxy consolidate had not actually consolidated the dirt and grit deep in the check (even though our standard practice was to keep applying the liquid epoxy until no more soaked in after 10 minutes). We set up side-by-side comparison field tests to determine which methods gave the best performance. Results by the end of the 1990s (and continuing today) reveal that clearing out all the debris and dust (and scraping the inner sides of the checks to bright wood) gives longest term durability.

Re: Fill Sill Weather Checks (draft)

Posted: April 21st, 2012, 4:30 pm
by jay treiger
Greetings John,
There is nothing like a few decades of field study to refine a technique. I do indeed attempt to remove as much debris as practicable previous to any application. I use System Three resin. They supply to the boat building/repairing trade but will sell to the general public including detailed instructions with every order. I have used the reheating technique on other (hardware store type) epoxies with similar results. I now completely avoid trying to speed set epoxies using the heat method as it leads to poor adhesion. I think you have already covered this. Partially cured epoxy will pull out of holes when scraping, and partially cured epoxy does not respond well to heat removal as it is still too sticky. I wait for it to cure before trying to remove it On steep sills I usually apply a duct tape dam on the lower edge to minimize waste and cleanup. Duct tape is able to resist the flow of a very fluid product that will dissolve other tapes. I use duct tape in most epoxy fills as either side 'releases.'without sicking to clamps or wood blocks

Re: Fill Sill Weather Checks (draft)

Posted: August 25th, 2012, 6:07 pm
by Paul Marlowe
Some recommended tools for damaged wood prep especially when repairing sill checks are a 90 degree mechanic pick/O-ring puller, HEPA vacuum and a triangular headed scraper. The scraper opens the check slightly if needed and the pick allows you to get deep into the check. The vacuum removes all the deep debris and helps control dust if used adjacent to each tool.

Epoxy consolidant is best applied until fiber saturation (no more absorption) is reached when possible. This could take 1-2 hours or more, depending on the porosity of the wood and the viscosity of the epoxy. Working multiple sills at once increases production. After this process is complete the adjacent wood face will be somewhat wet if there are lots of checks close together. I have not noticed this being a problem as it relates to trapping moisture, provided good techniques and appropriate epoxies are used. Every step of the process is important. Be sure to contain the epoxy to the sill only and be careful not to have a bleed out into the wall below or onto surrounding fabric such as masonry. Sometimes the spec. is not to remove all the paint but to fill the checks. In this case as with removing paint to bare or almost bare wood sills, it is ideal to prepare the substrate as if you are preparing the surface for paint, before applying any epoxy. As mentioned by John Leeke, a squirt bottle is ideal, but a 1/2" chip brush can also work to fill checks somewhat neatly. Patience is very important while working with epoxies.

If after consolidating you feel you still would like more epoxy resin to penetrate the walls of the checks, some systems allow you to create a slurry with the filler. Applying a slurry consistency allows more resin to soak into the wood. This increases the bond and flexibility of the epoxy. To finish off, thicken the filler and top coat flush to the surrounding surface unless you need to span low spots and wish to achieve a smoother surface. In that case you will have to have consolidated that entire area as described, prior to applying the epoxy filler. The consolidant and its application is very important to the durability of the repair. Either way the adjacent wood will have at least a thin film of epoxy over much of its surface.

During the curing process more of the epoxy liquid might continue to soak into the substrate causing slight but noticeable low spots in the checks once the epoxy is fully cured. This is sometimes acceptable, but if not, a second coat of just the filler can be applied. Allow the epoxy to fully cure in order to achieve the best results when tooling. Prime with water based primer to protect the epoxy from UV light. Scuff sand and apply two coats of 100% acrylic paint.

Re: Fill Sill Weather Checks (draft)

Posted: August 26th, 2012, 7:09 am
by johnleeke

Thanks for your thoughtful response.

I also sometimes leave the paint in place if it is in good condition. I have added that to step 1. , and added your name to the Contributors for this standard.

Re: Fill Sill Weather Checks (final)

Posted: August 1st, 2013, 11:04 am
by johnleeke
Some workers are having difficulty when the wood separates from the epoxy after doing the repair. I remember having this problem myself in the late 1980s, on a couple of projects where we did not take the time needed to dry out the sills enough. At that time we set a "shop standard" for dryness of the sill when using this method.

The wood has to be dry. The repaired checks can opened up because the wood continues to dry out and shrink away from the cured epoxy, especially if the repaired sill had a high moisture content to begin with and is left without paint for days or weeks after the repair is done. To keep the checks from opening up after wood-epoxy repairs dry the sill down to 5% less moisture content than the wood will be normally. For example, if the other painted exterior woodwork on that side of the building stabilizes at 15%, then let the sill dry down to 10%. That's 10% all the way through. Then do the wood-epoxy repairs and get the paint on before the wood dries some more and shrinks away from the epoxy. The wood will slowly (over a few or several months) go back up to 15%, putting the filler in compression, preventing opening up.

Checking window sill for moisture content with a pin-type moisture meter. The pins are insulated on the sides so the moisture reading is taken just at the tip. This way the moisture can be measured in layers to determine the difference between surface drying and drying deep within the sill.

Perhaps moisture contents should be made a part of this standard. What do you think?

Re: Fill Sill Weather Checks (final)

Posted: August 9th, 2013, 12:24 pm
by johnleeke
This method requires protection with paint, and maintaining the paint to limit wood movement that can lead to separation at the wood-epoxy interface and buildup of moisture that can lead to decay.