Black Spots Under Clear Coat on an Exterior Door

      Finishers troubleshoot finish failure on an exterior door sprayed with clear lacquer. September 27, 2008

Subject: Solid Douglas Fir exterior door.
Problem: Homeowner installed a new unfinished Fir door/side lights to a north facing side of house. Every weather condition blows in from the north and door was not sealed until several weeks later. Two summers ago, I was hired by the painting contractor to seal the exterior and interior of the door with the same interior Cloverdale clear coat lacquer provided by paint contractor.

Four months later the homeowner contacted me to inform me that the lacquer failed and the door looked horrible on the outside. We made arrangements for me to paint the exterior of the house plus fix the front door in the summer of the following year. The lacquer coating continued to fail through the fall, winter, spring and summer until I came around in late August to address it.

I stripped the lacquer off with "Dad's Easy Spray" stripper, washed it with paint thinner, sanded completely every square inch of the wood, prepped for spray application and applied two coats of water-base "Aqua Guard" clear coat.

Problem: There now appears to be "black pepper spots" over the door under the finish and I am at a loss as to what these spots are. I told the homeowner it could be either mold which the top coat is not crackling or lifting in these spots or it is sap. These spots are not showing on the interior of the door only showing on the outside. Does anyone have any suggestions as to what has happened?

Forum Responses
(Finishing Forum)
From contributor R:
Did you use any steel wool with that stripper? Once in awhile a water based stripper or top coat will react to the steel wool thatís snagged in the grain. Even the best of prep-work can have some residual steel wool. The water and the steel wool donít get along to well and what youíre looking at could possibly be rust.

Just a thought on the pepper spots. It might be leftover mold staining from the year or so that the finish was failing. Is there a reason you didnít use a tougher or stronger finish that could withstand the elements, lacquers donít hold up very well on exterior doors.

From the original questioner:
Thank you for your response. I am having a meeting with the Paint Store rep to take pictures and get some other 'minds' involved to figure this out.

Steel wool was used sparingly but what I mostly used were stripping brushes that look like tooth brushes only with copper bristles. Most of the removal of the old lacquer finish was stripped with a 2" metal edge and the detail was cleaned with the brushes then washed with paint thinner then sanded.

Lacquer was not my choice to use and I even warned the homeowners that I did not feel comfortable "sealing" the outside of the door with it. As this job was not my contract but was hired as a sub-contractor to "seal" the door, the contractor had me arrive to his paint shop wherein he gave me the lacquer to use on the door. Knowing this contractor and wanting to show a willingness to do what I'm told without haggling over technique, products and second guessing his knowledge, I needed the work at the time and also knowing I was also having to deal with his pride, I simply took the lacquer he gave me and did the job.

I made it very clear to the homeowners if it was my contract I would not use lacquer on the exterior but a product made specifically for exterior. It was a chance they were willing to make and maybe they had their reasons for having me continue, I don't know. It just turned their $7000 Douglas Fir door into mess.

From contributor G:
You said unfinished fir, like Douglas fir or Frazier fir? These are pines with sap and maybe it is sap oxidizing through and/or with the failed films. No matter how well kiln dried it is down to say 5%, weather changes that extreme will swell and shrink and squeeze leftover sap back to the surface. Strip, wipe with vinegar, wipe with alcohol, stain to desired color and Man' O' War spar varnish is what "nearly" every boat owner and exterior door finisher here in South Florida uses.

I left out sanding prep, most of us know that, leave out the steel wool. Yearly touch up required but the stuff wonít fail on you. If I walked up to it not knowing that's what I would try. I could be off base. Contributor R had a good point with the steel wool. If the door wasn't cleaned off well, then the new coats will pull tiny particles together into clumps, but oxidizing to black not red is sort of weird. Also pigmented 100% acrylic latex is used a lot down here for exterior work.

From the original questioner:
I was given two product suggestions to use. Spar Varnish was one but was told "this product is old school" and Aqua Guard was highly recommended

From the original questioner:
Here's the problem. The rep and I believe it to be mold or fungi. The problem now is how do you fix it and provide a guarantee that it won't happen again?

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From contributor P:
You need to treat the bare wood with a fungicide before refinishing. Bleach usually works, but will lighten the color of the wood. Some exterior finishes have fungicides built in. I remember buying a separate fungicide additive where you add a few drops to your finish. Assuming it's still available, it should help inhibit mold from growing under the new finish. Based on the picture, that looks like a bad location for a wood door - too exposed. It'll need to be refinished every year or two. I wouldn't warranty the finish on it, no matter what you use.

From contributor R:
Get your hands on a moisture meter and check the wood for moisture content. That door looks like it has been soaking up water for a long time. If the MC is higher than 10 - 12% (and that is high) they will have to dry out before you put any finish on. I would guess the steel wool caused the black marks, if they won't wash off with detergent I would use a mild oxylic acid solution.

It looks like it gets a lot of direct sun also, Spar varnish will not hold up. I would use Sikkens Cetol1 and Cetol23. The first coat of Cetol1 must be brushed on then the second and third coats of Cetol23 can be brushed or sprayed. The beauty is you don't have to strip the door in a year just apply another coat.

From contributor B:
I think itís mold and Contributor R's right also. That door has soaked up a lot of moisture. Did you seal all surfaces to include the edges and top and bottom?

From the original questioner:
It is impossible to seal all sides as the door is assembled, stapled together at the factory and installed. In order for a complete seal, it would have to be taken apart or caulked and painted.

From contributor B:
I'm talking about the bottom, side or top edges, or any possible way for moisture to enter. And normally, on the average, a finish for outside exposure last about a year before re-doing. The finish needs to be exterior rated not an interior type finish.

From the original questioner:
The installers sealed it but there are many joints all over that weren't. The second product applied after stripping off the lacquer is "Aqua Guard". It is an exterior product. It is my opinion that Fir should be painted. Natural finish woods in wet conditions should be cedar.

You have to take it apart to do it right. Lacquer?

From the original questioner:
To me, lacquer for exterior is just plain wrong. The rep mentioned to me that there are a few big name paint contractors in town that exclusively use lacquer for exterior and have no problem using it.

From contributor D:
Having tried using lacquer for exterior do you agree?

From the original questioner:
Lacquer does not handle moisture at all. We all know what happens on interior window sills that have been lacquered and the condensation pools on them after running down the glass but for some reason there are contractors that use it on the exterior.

From contributor G:
Drill small holes in affected areas about 1.5-2.5 inches apart. Use a heat gun on low setting or hair dryer to dry wood. Then warm (not too hot) some GET-ROT or RX which are two part epoxies and inject into the affected areas. That should keep the end grains from soaking up moisture again.

The marine industry is a great place to research when moisture abatement is what your after. Problem you have is achieving a pleasing looking finishing without monster buildup, and discoloration after time. I recommend epoxy to stop moisture intrusion, then top-coat with something from the marine industry. Getting that end-grain or anywhere that wood meets metal to be water tight is going to be a chore, but you can fill the holes without any severe alterations to the overall look. Since it is such a soft wood, you may be able to get by with just a large syringe driven directly into the wood, but holes are better.

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

Comment from contributor S:
Fir doors have a lot of pitch in them. The newer versions of veneered fir doors even have more. What I have found is that when the door heats up and pitch comes through the surface and your finish, itís a perfect pathway for mold/bacteria to travel into your door and turn black. The black spots are underneath the clear finish and not superficial so bleaching is not effective. The pitch creates a perfect petri dish that the bacteria thrives in. These are not water stains. The problem with using a spar varnish is that eventually it will need to be stripped. The cost of stripping the door is cost preventative in most situations. If itís a veneered door itís most likely not possible and would need to be painted. Most door manufactures will guarantee the door against pitch coming out but not water going in. As soon as you see pitch and youíre within your warranty period (one year) call your supplier.

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