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30 May 2010

Wood Door Finishes

Last year, while updating our wood door specifications, I became a little confused while trying to figure out the finish systems specified in AWI/AWS and WDMA standards. In the past I had specified wood doors using AWI standards, but after investigating the relevant standards, I changed our wood door specifications to use the WDMA standards because they are more specific to wood doors. However, because some literature refers to AWI and some to AWS standards, and some refers to current standards while others refer to old ones, I created a conversion table so I don’t have to go back to the standards whenever we get shop drawings.

Standards of the Architectural Woodwork Institute (AWI) and the Wood Door Manufacturer’s Association (WDMA) have been similar for a long time, and in most cases, a door that meets Premium Grade standards for one will meet them for the other. Perhaps the most important exception is face veneer grades, but I'm going to limit this discussion to finish standards.

Architectural Woodwork Institute (AWI)
The AWI Quality Standards, 7th Edition, issued in 1999, had a table of finishes with designations like TR-1 and OP-3. Each numeric code corresponded with a type of finish, e.g., catalyzed lacquer, and the prefix indicated transparent or opaque.

The 8th Edition, issued in 2005, contained major changes to the table. The convenient type designators were eliminated and finishes were referred to by names. The order of finishes also changed, with the former TR-0 system moved, to between what had been TR/OP-4 and TR/OP-5. The system table was prefaced with this note: “This table does not represent all possible top coats. Other options will be found in the text on the following pages.” However, it appears from those pages that the main difference was splitting one category into two sub-categories.

In 2009, AWI joined forces with the Woodwork Institute (WI) and the Architectural Woodwork Manufacturers Association of Canada (AWMAC), and the three organizations produced the Architectural Woodwork Standards (AWS) Edition 1.

The new finish table has thirteen finish systems. The systems are the same as before, but only one of the designators is the same as those used in 1999. The rest increase by one or more numbers as old systems were split into two new ones. For example, the 1999 catalyzed lacquer became pre-catalyzed lacquer and post-catalyzed lacquer.

National Wood Door Manufacturers Association
The 2004 WDMA describes sixteen finishes. All of them, with one exception, appear to be identical to those in the 1999 AWI standards. In addition to brief descriptions, they have extended information for the three types of finishes that are most often used by door manufacturers: TR/OP-2 pre-catalyzed lacquer, TR/OP-4 conversion varnish, and TR/OP-6 catalyzed polyurethane.

I have all of the AWI/AWS standards referred to above, but I have only the current WDMA standards, so I cannot comment on the history of WDMA standards.

Finish System Properties
To make things even more interesting, the method of evaluating system properties has not been consistent. All of the performance tables use a 1 to 5 rating scale to rate various characteristics such as repairability and chemical resistance. All of the evaluation criteria rank 1 as low and 5 as high, except for the 1999 AWI standards, in which 1 is excellent and 5 is poor. The advantages of starting with 1 at the low end are obvious - total scores can determined by simply summing the scores of individual characteristics - and the new AWS standards again use that method of scoring.

Finish System Comparison
Assuming wood door manufacturers most often use one of three finish systems, let's look just at those.
The overall performance scores of the three finishes increase from left to right, the notable exception being repairability, where the order is reversed. In simple terms, the higher the performance of a finish system, the harder it is to repair, which makes sense.

The chemical resistance performance also increases from left to right, with respective scores of 86, 114, and 118. The differences between conversion varnish and catalyzed polyurethane are a marked increase in resistance to 77% sulfuric acid and a small difference in resistance to 10% TSP, both in favor of catalyzed polyurethane.

I sent these findings to both AWS and WDMA representatives, and was told that my conclusions are correct. The WDMA added that the system most often specified is TR/OP 6 - Catalyzed Polyurethane, which provides the highest performance properties of production finishes, and that they normally are roll coated using a high solids, VOC free material, then cured by an ultraviolet (UV) process.

WDMA also indicated that they are updating their standards, and one of the areas being addressed is the finishes section. Look for new standards to be issued in the fall of 2010.


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