Turn Rotten Wood into Gold: Alchemy, or Abatron?
You decide, as we discuss this article at the Historic HomeWorks Forum.
Following is the original manuscript for the article titled, Making Sense of the Mercurial Epoxy, published in the Sept. 2004 issue of Old-House Journal as their Conservator feature.
Epoxy resins are now used to produce a wide variety of products and as a basic material for building repairs, especially the repair of deteriorated wood. Other resins are also used and sometimes confused with epoxies. Some epoxy products are touted by their manufacturers as a "Miracle Cure" that turns rotten wood into gold. While others are denigrated by the architectural conservators as an "inherent vice" of immoral depravity that does more damage than good. What is epoxy? Is there a practical middle ground where this chemical repair method can be used to save exterior woodwork successfully?
Photo (right), Mixing epoxy paste filler. (photo by: Steven Swiat, Preservation Carpenter)
Epoxies are a type of plastic made up of two liquids, an epoxy resin and a curing agent called a catalyst or hardener. When the resin and hardener are mixed together a chemical reaction takes place and it hardens into a solid mass. When the two components are mixed, single molecules (monomers) of the epoxy resin and the curing agent combine to form chains of molecules (polymers). As the mixture "cures," the chains grow longer, the solution gels and then hardens into a solid mass. The characteristics of cured epoxies (such as whether they are firm or flexible, or resistant to heat or chemicals) depend on which epoxy monomers, curing agents, solvents, and fillers are added. Many epoxy systems also contain additives such as plasticizers to make them more flexible, organic solvents to make them more spreadable, and fillers such as sand to add bulk and reduce costs. Pigments are added for color and fibers such as Fiberglas(tm) or chopped cotton change to improve strength characteristics.
Epoxy materials are very adaptable. The chemists who design and formulate epoxies can give them a wide range of pre-cure and post-cure characteristics. The pre-cure mix can be thin as water to penetrate porous materials, or thick and viscous to stay put on vertical surfaces. The cured epoxy can be nearly as hard and brittle as glass or almost as soft and flexible as rubber bands.
Photo (right), Using epoxy paste filler to fill nail holes and as gap-filling adhesive in clapboard repair. (photo by: Steven Swiat, Preservation Carpenter)
Even the rate of cure, from several minutes to several hours, can be adjusted to meet very specific application needs. ConServ's Flexible Consolidant 100 product takes hours to gel which helps it penetrate deeply into porous wood. Edison Coating's Flashepoxy 580 products are fast-curing for floor coating and repair situations where the area must be put back into service as soon as possible. This is a range of products formulated to permit the selection of the desired rate of cure that can match the anticipated temperature and conditions during application that can range from sub-zero winter to hot summer weather.
As the mixture cures the reaction also generates heat. When large amounts are mixed or thick sections are treated heat builds up causing faster setting and even more heat buildup. This building cycle of heat can make it set sooner than expected, or even cause a fire. The repair must be designed and products selected to control the heat and give the fast or slow cure needed.
After several hours or a few days the epoxy reaches its final strength, flexibility and hardness. Depending on the manufacturer's formulation, the epoxy will be more or less flexible. Flexibility is a key characteristic in the design of a wood-epoxy repair, as it must complement the strength characteristics of the surrounding wood.
Photo (right), Using epoxy adhesive to glue up porch column plinths. Credit: John Leeke, Preservation Consultant
Epoxies bond exceptionally well to a wide range of materials making them well suited as adhesives. Good bonding also means epoxies are useful for making composite materials. An effective repair can be created by layering epoxy with a fabric and bonding the resulting composite to surrounding components, or by mixing the resin with powdery fillers or chopped fibers to make a paste that fills voids.
All this adaptability makes epoxies well suited for matching the characteristics of traditional building materials and systems that need repair. Stone, glass, concrete, terra cotta, and wood can all be repaired with epoxies. Epoxies for wood repairs are formulated especially to match some of the strength and flexibility characteristics of wood. Don't confuse these epoxy materials with epoxies formulated for other uses such as 5-minute adhesives, bar top coatings, or paints.
Several manufacturers have specially formulated epoxies for such repairs and offer them in kits of the various materials needed for easy mixing and use. (See Resources)
Epoxy Consolidant: Consolidant is a liquid formulated especially to soak into fibrous materials such as wood. The amount of penetration depends mainly dryness of the wood. Other technical considerations such as capillarity, interfacial tension, molecular size, temperature, consistency and gel time also play an important role.
Epoxy Primer: Primer is liquid formulated to prepare a surface for good adhesion of an epoxy paste filler.
Epoxy Paste Filler: An adhesive paste is made of a two-part liquid epoxy very similar to consolidants. Powdery thickeners are blended in to make a paste that ranges from the consistency of mayonnaise to stiff mashed potatoes. Other fillers give the cured paste the strength and flexibility characteristics of wood. You can even formulate your own paste by starting with epoxy resin and adding familiar materials like sawdust and a little corn starch, but commercially prepared materials are much more consistent and reliable.
Cause: Wood often decays when moisture builds up, cannot dry out, and when fungus consumes the wood as a source of food. The source of the moisture must be determined and controlled. If a window sill is decayed due to a gutter leak above, the gutter must be fixed along with the sill, or the sill is likely to decay again. Consider the climate. Fungal decay is less likely in the arid Southwest, but highly probable in humid South. Even the damper micro-climate on the north and east sides of a building may promote decay, while the south and west sides are kept drier by the warming sun.
Methods: There are two common methods of wood-epoxy repair; decay consolidation and decay removal. Decayed wood is softer, weaker and more porous than surrounding sound wood. The porous decayed wood is saturated with liquid consolidant, which then hardens within the wood. It is important to assure that absolutely all of the decayed wood is saturated with consolidant right down and into the surrounding sound wood. This is difficult to do since the interface of the decayed and sound wood is hidden. Another method is to removed all of the decayed wood. The exposed sound wood surface is primed with an epoxy primer, then the void is filled with an epoxy paste filler. It is easier to get more effective results with this method because the newly exposed surface of sound wood is much more consistent that the interface of the decayed and sound wood interface in the consolidation method.
Decay Consolidation Method:
A. Identify soft, porous decayed wood.
B. Drill holes in decay area, add preservative, dry out.
C. Saturate decayed area with epoxy consolidant, fill holes and voids with epoxy paste filler.
Decay Removal Method:
A. Identify soft, porous decayed wood.
B. Remove decayed wood, add preservative, dry out.
C. Prime sound wood surface with epoxy primer, fill void with epoxy paste filler.
Failure: Wood-epoxy repairs can fail when moisture is trapped in sound wood right next to the repair, causing further decay. This is likely to happen because the epoxy is very impermeable to moisture. The moisture cannot migrate through the wood and escape as it usually does. To prevent failure the sound wood next to the repair area is often treated with chemical preservative, as a part of the repair.
Reversibility: Treatment reversibility is a key requirement of building conservation sometimes required for work on important historic buildings. If a treatment cannot be removed without casing damage to historic material, then it cannot be used in the first place. These epoxy materials are formulated to penetrate deeply and adhere tenaciously. This is what makes them so effective from a practical point of view, but it also makes the repair difficult to reverse. While wood-epoxy repairs can be designed to be reversible, this is not the way they are commonly done.
It is recognized that these wood-epoxy treatments may not be as reversible as demanded by current conservation philosophy. But, very often a balance can be struck between the shorter maintenance cycles of more reversible treatments and the longer maintenance cycles of less reversible treatments. It is particularly important to achieve a longer maintenance cycle on woodwork in remote locations, such as steeples and high towers, due to the high cost of access. This not only reduces the cost of maintenance, but also has been demonstrated on at least one national landmark building over the years to result in the preservation of more historic fabric. This balance between reversibility and sustainability must be kept under consideration, always leaning toward reversibility with important historic buildings.
Keep in mind that there is nothing magic about epoxy that automatically makes it better. Traditional repair methods such as wood dutchmen or splicing, and modern methods such as part replacement should always be considered along with wood-epoxy repairs. Select the method and materials that meet the needs of your repair and the goals of your situation.
I have noticed recently that many homeowners and tradespeople (and even a few writers and editors of national publications) are using the word "epoxy" in a generic sense to mean any resin used in the repair of wood, even when the resin is not epoxy. I suspect what they really mean is the method, not necessarily the material used. The method is to repair wood using a liquid or paste that hardens onto or into wood forming a “composite repair” where the resin is intimately associated with the original wood and added wood or other reinforcing materials. Strictly speaking, "epoxy" means a specific chemical system. Some wood repair products use resins and binders that are based on other, very different, chemical systems. (See Other Materials below) Perhaps "wood/resin composite repair" would be a more accurate phrase to describe these repairs, since it encompasses a wider range of materials.
The following repair products use resins and binders that are based chemistry systems other than epoxy.
Bondo(tm) Auto Body Filler(tm): Auto body workers know this product well. It's used to fill dents in sheet metal. It is hard and inflexible when cured. When used on wood it is likely to loosen and fall out due to the inevitable movement of exterior wood caused by changes in moisture content.
MR. MAC'S(tm) Wood Fix: This wood filler and repair system is based on a special cement, acrylic latex polymer, fillers and fiberglass cloth. The manufacturer says its high adhesion and flexibility prevent loosening due to wood movement. Unlike epoxies, it can be applied to damp wood and when
cured allows water vapor to pass through thin sections. The cured product is alkaline which, according to the manufacturer, prevents wood decay but in my opinion may also cause adhesion problems with alkyd resin paints.
Minwax(r) Wood Hardener and Wood Filler(tm): Wood Hardener is a consolidant that is 22% poly-ketone resin in an acetone-methene solvent. The solvent promotes penetration, but only the resin remains in the wood, the rest evaporates. Wood Filler is a two-part paste made of polyester resin and fillers. According to a Minwax Technical Service representative the filler "cures rock solid" and is not flexible and has little give and take with the wood as it expands and shrinks due to changes in moisture. In fact they recommend using nails with the heads acting as keys to hold the patch in place when it loosens. I see this as a potential problem in exterior woodwork repairs because the loose patch could trap moisture behind it causing decay. It would be better to have the patch fail by falling out so it can be easily recognized that maintenance to the repair is needed. Drying and curing times for these Minwax products are faster than epoxies, but I have some concerned about how long the repairs will last.
In fact, the jury is still out on long-term performance for all these other products since they have only been in use for several years. Epoxies used for wood repair have a track record stretching back more than thirty years. But, who knows how long they will last? Years? Decades? Centuries? Only the test of time will tell for sure.
Do not interpret the above to mean that these other products are "bad." As you know, in my book there are no "good" or "bad" products. All products have certain characteristics and it is our job as old-house mechanics to know what those characteristics are so we can select a product that give the result that is needed. In fact, the design for the repair, the method, the procedure and techniques used to apply the product have a far greater effect on how long a repair will last, than which product is used.
I demonstrated this on a training project back in the mid-1980s by using a wad of bubble gum, dust swept up off the floor, a candy bar wrapper, a little nail polish and laundry detergent to repair a decay pocket in exterior woodwork. As of 2004 that repair was performing as well as the epoxy repair right next to it. Success is found in the method, not in the product.
After four decades of repairing buildings with both epoxies and traditional methods, I believe our confusion about wood-epoxy repairs lies in the product-oriented approach provided by the building construction industry. When we think rationally to decide first if our goal is to preserve valuable historic material, or to just make a practical repair; if the repair should last 100 years, or 10 years; if we will use a traditional wood-dutchman repair, or a high-tech wood-epoxy repair; if we will hire an experienced craftsperson, or do it ourselves; and last, if we will use this product or that product; then we have our priorities straight and our repairs will be successful.
John Leeke is a preservation consultant in Portland, Maine. You can contact him at: 26 Higgins St., Portland, ME 04103, 2l7 773-2306, www.HistoricHomeWorks.com
PC Woody Epoxy Products
Protective Coating Co.
Wood Care Systems
Mr. Jim Renfroe
751 Kirkland Ave.
Kirkland, WA 98033
Rotfix, consolidant & primer
Sculptwood, paste filler
ART, Advanced Repair Technology
Mr. John Stahl
PO Box 510
Cherry Valley, NY 13320
Primatrate© flexible epoxy cell bonding primer
Flex-tec HV© elastomeric epoxy wood repair compound
5501 95th Ave.
Kenosha, WI 53144
WoodEpox, paste filler
PRG Preservation Resource Group
P.O. Box 1768
Rockville, MD 20849-1768
800 774-7891 sales
301 309-2222 tech support
JP-146 Epoxy Consolidant
FM-250, powder for mixing with JP-146 to make a paste filler
ConServ Epoxy LLC
Mr. Paul Marlowe
PO Box 454
Northford, CT 06472
phone (203) 484-4123
fax (203) 484-2398
Con Serv (tm) Flexible Consolidant 100
Con Serv (tm) Flexible Patch 200, filler
Smith & Co. Restoration Products, Inc..
5100 Channel Ave.
Richmond, CA 94804-4646
Fill-It Epoxy Filler
Clear Penetrating Epoxy Sealer
26 Higgins St.
Portland, ME 04107
"Wood-Epoxy Repairs for Exterior Woodwork"
This booklet covers how wood decays, epoxy materials and methods, tool kits, safety, and alternate materials and methods. 33 illustrations, 4 methods and step-by-step procedures, 31 pages. Available online at:
Turn Rotten Wood into Gold: Alchemy, or Abatron?
You decide, as we discuss this article at the Historic HomeWorks Forum.