Decks, Patios, Porches and Docks - Black staining of redwood railing under clear coat

About two years ago, we replaced the railing around our deck with redwood railings (horizontal and vertical members). Above the Sonoma Coast we get scorching and sun for 10 hours a day on this deck that faces south. Then in the rainy season we get blasted with wind-driven rain for months on end. After we installed the new railing, we let it age for a few months before applying a clear coat with UV protection to the entire thing (can't remmber the brand). Now it's time to refresh the clear coat. Especially the top ledge (2x6 horizontal) finish is deteriorated and blackened; we will apply a solid color stain to that. But the vertical components (2x2 redwood) still look pretty good, and we'd like to keep them natural-looking with the same clear coat with UV protection. When I try to clean the black stuff off them (presumably mildew), I have modest success. I've tried commercial deck cleaner, soap/water, soap/water/bleach. If I scrub too vigorously, I am just removing the clear coat, which I'd prefer not to do if possible. Much of the surface stuff comes off. But there also appears to be black stuff (not sure if mildew or extractive bleeding(?)) under the surface of the old clear coat. Is that possible, or am I just not getting the surface clean enough? Can I get this sufficiently clean for a new clear coat, or will I have to power-wash the old clear coat away? Or should I just throw in the towel and use solid stain on the entire thing? Any ideas welcome! Thanks, Jason

What type/brand of clear coat did you use?

Some deck sealers require removal [either chemically or weathered away] before they can be recoated. Pressure washing can be the best method for prepping deck/railing for recoating but care must be used so you don't damage/mar the wood with excessive pressure.

Generally a semi-transparent stain will last longer than a clear sealer.

The black is caused by the tannin acid in the redwood that has gotten wet. You're not going to remove it, no matter how hard you try.

And you are talking to the wrong person about how harsh your weather extremes are. There's no way the Sonoma coast will ever see 115 to 118 degrees in the summer, or 10 to 15 degrees or so in the winter like Shasta County does. The amount of rain is probably about equal -- 45 to 50 inches -- but your's DOES take a few more months to happen!

Marksr said use a semi-transparent stain. It's either that or a solid satin.

Hey lefty and mark. Thanks for both your comments and suggestions. I wondered whether it was a tannin problem or not. And you are right, lefty, Redding weather is even more extreme than here! Question: do you guys think a semi-transparent stain would cover the black stuff, or should I just resign myself to solid stain at this point? Thanks, Jason

Semi-transparent will hide more than a translucent or toner will but there will be some of the black show thru. Hard to say without seeing it.