Polyester and Epoxy Wood Repair Fillers in Conservation Work

Tips on whether epoxy or polyester wood repair formulas are sufficiently authentic for conservation work, and some tips on how to use them. October 26, 2007

I know that conservators approve and use epoxies for doing certain exterior repairs. Do they also approve of the polyester auto body fillers for doing exterior repairs?

Forum Responses
(Finishing Forum)
From contributor A:
I'm not a conservator by any means, however Bondo should only be reserved for cosmetic repairs to wood. Polyester resin does not bond well with wood, especially when thickened. For instance, if you are going to bond fiberglass to plywood with polyester, it is necessary to thin the first coat with styrene to soak it in. Then thick poly and matt (holds gobs of resin), then cloth with more resin. Whereas with epoxy, you can bond fiberglass cloth directly to plywood in one shot.

From contributor D:
I am a historic furniture conservator. But I was first and still am a contemporary finisher. I try to bridge the gap that both sides seem intent on creating. This is and has been my opinion/position on the ethical and practical question you have raised. I have stuck my neck out in the conservation community more than once, and waited to get slammed, but never did. I got nicknamed the Bond Girl. This is why. My testing experience has been that it works very well in many situations and that there is nothing irreversible, detrimental or ethically wrong with the use of this product. I have been using it a long time. I have used it mostly for interior based wood projects and artifacts, however done properly, I believe it to be and have suggested it to be a very viable option for exterior wood repairs also, historic or not.

Abatron and such epoxies have been used frequently and have been the preferred method of historic preservation guidelines when at all possible to repair exterior wooden elements versus the last resort of replacement with new (for example, repairing a window sill by retaining as much original material as possible, rather than making a whole new window).

Specifically regarding polyester fill, a.k.a. Bondo, I have used it for many years, even on historic pieces (with a barrier coat bridge and client approval). I have done repeated ad hoc Oddy tests with Bondo polyester in various stages of cure time under several different scenarios and have found zero adverse affects. Once cured there was no out-gassing in test results. No tarnish on silver. I also discussed this at length with one of the chemical technicians of the company which produces the commonly known automotive filler Bondo after receiving their MSDS and tech data sheets. Not that I believe what anybody tells me, but my test results gave me sufficient confidence that I was on solid ground on this one. Most conservators just don't like the sound of it. Too bad. It is fast, inexpensive, and therefore more preservation can be done with the available funds. Now, I do use it differently in different circumstances, depending on the materials that are involved and the environment it will be subjected to.

Indoor environmentally controlled artifacts will have less movement of the wood, and I will mix a batch accordingly. When I want to make Bondo poly fills in a wooden artifact that may have more movement, expansion/contraction of the original materials, I will adjust the formula of the fill so that it will stay more flexible and not bust out or cause stress elsewhere in the object, causing damage.

I also use it to make quick cheap flexible molds to reproduce missing elements. My opinion is just based on my low tech testing, but the Getty nor anybody else has slammed me for publishing it. Anyone out there know how I make it more flexible, or want to know? What are the particular circumstances that made you ask the question?

From the original questioner:
I certainly am in agreement both with epoxies and polyester repairs. For years, most conservators dreaded when anyone even mentioned these two filling mediums. Most conservators would not even mention epoxy for doing repairs; it took years to get some of them to admit to it.

I recently had a discussion with a shop that is now working on a very old structure. They are doing repairs on lots of the interior and some exterior work. The question came up about using both of these fillers. I said to use them whenever they needed to fill certain damages, but only after they had removed the damaged soft wood, and had solid wood that they could fill and shape. I also feel that if new wood is added, they should bond the new and old woods together to strengthen the repairs.

My reason for posting was to see how the conservators are thinking nowadays. Thanks for the posts. I guess it still depends on the conservator who you ask.

From contributor D:
Yes, conservators are an interesting bunch. Don't let it get to you. Anyone who thinks they know it all clearly does not. Still, most of them are profoundly studied and ethical, and others are downright liars. It is not a licensed profession and anybody can call themselves a conservator and many do. Either way, as I have often said, ask 10 conservators the same question and you will get 15 different answers. It is not a textbook industry yet; we are learning the answers and science and finding new materials as we go. And things that were done and thought to be acceptable 25 years ago, we absolutely cringe at now. That is where the concept of reversibility came into play... because we don't know everything. Now we lean toward retreat-ability rather than reversibility. The simple truth is that when using a liquid consolidant to soak into and stabilize wormwood or waterlogged wood, we will really never be able to completely reverse this process safely and probably would never want to. We try to never remove original material, but there are rare occasions when small amounts are too fried to save successfully. But we do try to make all decisions of treatment design and material use on the basis of "do no harm."

The National Park Service has many Preservation Briefs which are published and can be accessed through their site. Complete guidelines and descriptions of the levels of repairs and materials are detailed. It details the current thoughts and standards of what restoration is and what preservation is and when and where and how.

From contributor G:
I'd like to know how you make the Bondo flexible.

From contributor D:
Bondo is made more flexible by doing the opposite of what logic would seem. Over-catalyze it. Experiment with different amounts and you will see. It will kick faster and get hotter as it is doing it, but the more catalyst, the more flexible it will be. I use different types depending on what I am using it for. Very fine white polyester automotive for some things, Golden Extra brand when I want a yellow background, and just plain old gray Bondo. You can color it with dry powder before you add your catalyst, and you can use different colors of catalysts too. Play with it and you'll find out everything you can do with Bondo. Have fun, but ventilate until it kicks completely, as the styrenes are off-gassing until the cure is complete.

From the original questioner:
One bad feature about polyester fillers is they cannot take stress. Using this filler on aged, dry rotted woods is not the intended purpose, which is basically metals or fiberglass. In many cases, for exterior restorations, you may want to consider epoxies.

Playing too much with these chemicals can get you in a lot of trouble down the line. Over-catalyzing causes the molecules to expand and then overlap other molecules, which eventually causes cracking after a full cure. Unless it's a one shot deal.

From contributor E:
After 10 years heading up the finishing department of an architectural woodworking shop, I formed my own hand finishing business, which includes a lot of touchup and repair. For big losses and gouges, I use wood epoxies including Bondo, tinted with powders to match the raw wood. For deep scratches, I use hard wax fill sticks, which I then grain with a fine brush. Years later, worn wax is easily removed and refilled, while epoxy and lacquer and shellac burn-in sticks have to be dug out and can damage surrounding wood.

From the original questioner:
I appreciate your post, but I was referring to exterior repairs done by conservators on treasured buildings. Normally, conservators only do reversible repairs. There has been a change and some conservators are using epoxies and polyester body fillers to do some repairs to stabilize the wood or other material from further erosion.

From contributor E:
One of my exterior repair jobs was on the old Pulitzer mansion on E. 73 St. in Manhattan - 13 pairs of French doors. After removing existing paint with Peelaway, and copious amounts of rotted wood, the project director had me use a clear 2 part epoxy that shaped and sanded well, followed by priming and brush painting. It took gallons of the stuff and 4 months to complete.

From the original questioner:
Conservators very rarely strip. They restore. They want to save all they can save on historic pieces, be they interior or exterior. Restoring is not always the same as conserving.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor R:

I have used polyester resin products for non-structural repairs on wood items for years. I have a couple tricks I use that can't be that original, but I don't hear of people using. For deep and irregular voids in wood that say has experienced dry rot or termite damage I will first get the area as clean as possible and then prime the area to be filled with a polyester resin such as a laminating resin. You can saturate an area and bind splintered areas that thicker fillers will not get to. After this has cured you have a similar surface that the thicker polyester based filler can readily bond to.

For larger areas, or those that you are concerned with possibly flexing or shifting, I have used fiber reinforced fillers that are a pain to work with but you end up with a reinforced patch. The real key to all this is achieving a finish that will stick to the finished product and last for an extended period. I have begun using alcohol based primers of late and am watching to see how they hold up over time.