Black Mold Spots Under Spar Urethaned Exterior Doors...?

Ms. Fix-It

Does anyone have any experience with black spots developing under the finish of exterior doors? They appear to be mold, and the customer showed me what was left of the Helmsman Spar Urethane that the previous contractor used on the doors when they were installed 3 years ago. She says they sprayed the finish on, although she doesn't know/remember how many coats they used. I have stripped the finish off, sanded it down, and when that didn't work, even tried bleach, but the spots remain. The grain is somewhat random, especially in the trim work where the problem is most prevalent. Has anyone run into this at all? My magic hat is quickly running out of rabbits...

__________________
those who cannot hear the music believe the dancers mad...

eastend

could they be mineral stains? have you tried oxalic acid?

Darwin

Those doors could be made of reclaimed wood. The 'spots' could be old nail holes.

Otherwise, moisture may have been present when the original person sprayed with urethane. He probably didn't notice.

Ms. Fix-It

I have never used oxalic acid before. Is it aggressive?

__________________
those who cannot hear the music believe the dancers mad...

MALCO.New.York

It is the Industry Standard "Wood Bleach".

The only other recommendation would be TSP (Tri-Sodium Phosphate).


Oxalic Acid is quite aggressive. It is also the Industry Standard Automotive Radiator Flush!!!!

__________________
Something to One may be Nothing to another!

Ultimate Wisdom---------
http://www.youtube.com/watch?v=OW-cnizLDEE

neolitic

What are they made of?
Some types of "mahogany"
have mold sort of inherent.
Were they sealed both top
and bottom?

__________________
Put your location in your profile!
(Sorry....it seems there really are dumb questions)

naptown CR

Try the Oxyalic acid it may be from metal caught in between coats if the original guy used steel wool between coats. I never ever use steel wool scotch brites in various grades or sandpaper only. You also indicated that the door hadn't been refinished in 3 years. I have owned a number of boats with brightwork ( Varnished wood) on them and the spar varnishes are pretty good HOWEVER I would start with 6-7 coats and do an additional mainterance coat twice a year. Then I discovered Marine Cetol. This stuff is probably the best most durable finish out there. There is minimal between coats work and three coats are sufficient depending on the level of sheen you want. Oh and this stuff is pretty expensive last time I bought it was a couple of years ago and i remember it as being about $40.00 a quart and worth every penny. If your customer only wants to recoat avery few years this is the way to go. Otherwise use a Valspar captains varnish and about 25-30 coats and it will look like a Hinkley varnish job and you get to go back twice a year and add a coat

Ms. Fix-It

They are mahogany, not reclaimed, though...

__________________
those who cannot hear the music believe the dancers mad...

Rehabber2000

I have only seen that with oil based under a water based and crayon...kilz or zipper or the only things that you have to put a few coats on to cover the problem and then paint...

Ms. Fix-It

The oxalic acid worked great to remove the spots. We used it in conjunction with an alkaline-based cleaner to neutralize the acid. Thanks for all the great insights!

Is there anything anyone can throw my way to help me insure that the problem doesn't return; ie. that no moisture gets trapped under the new finish? They are mahogany doors, and I will be using a cetol marine finish, and am planning on doing at least 3 coats. Any other pointers? The last thing I want is to get a phone call in 6 months from my customer that the problem has returned...

__________________
those who cannot hear the music believe the dancers mad...

neolitic

Seal top and bottom just as well
as you do the faces.
Get some kind of oil (stain
or clear) to penetrate at the
joints, where the surface finish
will surely fail with movement.
(I assume it's a panel door)
Hope that the problem is not
inside the wood itself.
Good luck.

__________________
Put your location in your profile!
(Sorry....it seems there really are dumb questions)