Porch Railing Coating Analysis

By Tom Pawlak — GBI Technical Advisor

My porch railing at home was badly in need of painting even though I had painted it the summer before. After just one Michigan winter, the paint had cracked and lifted. I wondered to myself if coating the top rail with WEST SYSTEM® epoxy would improve this poor situation and decided to turn the top rail into a small R & D project.

The top porch rail is made up of four separate pieces of 5/4″ x 6″ pressure-treated Ponderosa Pine. During the winter, water and snow on the rail melts and freezes many times. This stresses the wood-to-paint interface because the wood expands when is gets wet, making it difficult for the paint to stay attached. My plan was to apply epoxy to parts of the railing to compare its weathering resistance to sections that were treated only with paint primer over bare wood.

The railing is 25′ long. Some of it is shaded much of the year. Some is exposed to direct sunlight at some point each day. There were six different coating variables that I wanted to try. So, to be fair to each coating type, I repeated the test several times along the railing so any differences observed in durability were not caused by exposure variables. I decided to repeat each of the tests four times by segmenting the railing into equally sized one-foot long sections and repeated the primer paint control five times.

Before any coatings were applied, I scraped and sanded the entire surface to remove the majority of the old paint. I then numbered the one-foot long test segments one through six on the bottom of the railing to identify each of the following coating experiments:

1. Cover Stain Primer Sealer by Zinsser
2. One coat of 105 Resin/205 Hardener, thinned with 10% acetone
3. One coat of 105 Resin/205 Hardener, thinned with 15% benzyl alcohol
4. Two coats of 105 Resin/205 Hardener, thinned with 15% benzyl alcohol
5. One coat of 105 Resin/205 Hardener, undiluted
6. Two coats of 105 Resin/205 Hardener, undiluted

I applied the coatings on a warm (80°F) afternoon. I recoated segments that required a second coat of epoxy while the initial coating was still tacky.

The epoxy-coated segments were allowed to cure partially to a non-tacky but soft state before I applied the house paint primer (Cover Stain Primer Sealer by Zinsser). Normally we recommend that the epoxy be cured overnight, wiped down with water and sanded thoroughly before applying paint, but I was pressed for time as rain was moving in that night. This seemed to justify bending the rules, as did the fact that my dear wife, Mary, wanted her porch back to its intended look sooner rather than later. I felt comfortable applying latex house paint over soft epoxy because our graphic artist, had done it successfully several years ago. So I decided to include this as part of the test. I applied a single coat of latex house paint over the entire top rail a few hours after applying the primer.

I’ve always felt that it would be great if we could identify a way to repair damaged wood trim or wood siding with epoxy, and paint over it the same day. I know there are contractors who prefer to use epoxy for architectural restoration but are forced to repair with lesser quality materials because they can’t afford to have an employee come back to the job to paint another day. I believe this test shows that it is possible to repair with WEST SYSTEM epoxy and paint on the same day if the right paints are used. The primer we used for our experiment dries quickly and is ready for topcoat paint in one hour.

There are other house paint primers that we have tested in the lab that dry and adhere well to partially cured epoxy. We’ll give you more information on these in the future.

Coating descriptions and results

1. Cover Stain Primer Sealer by Zinsser—Cover Stain Primer Sealer by Zinsser is a popular interior/exterior primer often used by contractors because it dries fast and because topcoat paints can be applied one hour after the primer is applied.

Results—Three of five sections peeled in just over two years (covering three summers).

2. One coat of 105 Resin/205 Hardener, thinned with 10% acetone— Solvent-thinned epoxy was added to our test project because we receive lots of calls from people who use this approach. Normally we steer clear of adding solvents because doing so can degrade the epoxy and reduce its moisture barrier qualities. In this test, though, I wasn’t concerned about moisture barrier qualities. I just wanted to reduce the maintenance required in repainting my porch railing. Because adding volatile solvents to epoxy makes the epoxy less of a moisture barrier, I thought there would be less chance of trapping moisture in the railing. Remember, the backside of this wood railing is not sealed with epoxy and it readily absorbs water during the damp times of the year.

Results— Three of four sections did not peel from the surface. The section that peeled did perform better than the paint-primed section nearby. So improvement was realized even though some peeling was observed.

3. One coat of 105 Resin/205 Hardener, thinned with 15% benzyl alcohol—Adding benzyl alcohol to our epoxy makes it considerably weaker. It also makes the epoxy more flexible. In this situation, I didn’t mind trading off some physical properties as long as the railing’s appearance was good and repainting requirements were reduced.

Results—Three of four sections came through without any peeling. Like the coating that was diluted with acetone, the section that did peel still performed better than the nearby section that was just primed with paint.

4. Two coats of 105 Resin/205 Hardener, thinned with 15% benzyl alcohol—This was a mistake. I actually intended this test to include one coat of epoxy diluted with 15% benzyl alcohol and an additional coat of undiluted epoxy (105 Resin/205 Hardener).

Results—All four test sections came through three summers without peeling. As it turns out, my “mistake” actually performed well.

5. One coat of 105 Resin/205 Hardener, undiluted— I was fairly confident that a single coat of our epoxy would improve my paint peeling situation. I was curious to see how it would hold up against two coats of epoxy.

Results—All four test sections came through the test without peeling.

6. Two coats of 105 Resin/205 hardener, undiluted—I was highly confident that two coats of our unmodified epoxy would improve my paint-peeling problem. In the back of my mind, though, I wondered if it would trap moisture in the railing, causing other problems.

Results—All four sections came through two Michigan winters and three summers without peeling.

Conclusions/comments

You can see from the results above that the epoxy-coated sections outperformed the paint primer controls significantly. However, we need to be careful not to draw too many conclusions from this because we only compared the epoxy base coats against one primer paint. Some paints are more elastic than others and are better able to move with the wood and adhere to it over time. So, even though the epoxy-coated sections performed well compared to the one primer tested, other primers may be as good as or better than the epoxy as a base coat on wood to resist paint peeling. We will likely do more testing to investigate this further.

Even though the results of this test are encouraging, we are not suggesting that you coat the entire outside of your wood house with epoxy. Epoxy is an excellent moisture barrier and as such may not allow moisture from the inside of the home to escape. Even oil-based paints can be a problem that way. Latex paint allows the wood to breathe to some degree because it allows moisture to escape more readily though the coating.

There are times though when coating wood on specific areas of a home with epoxy does makes sense. In places where moisture entrapment isn’t an issue, I think the concept has merit. I can assure you that the sections of my porch rail that were coated with primer and are peeling noticeably now will be sanded down and sealed with a coat of undiluted epoxy before painting again.