Q: My redwood house is 40 years old and was never stained. It is weathered gray. Can I stain it now and if so what prep work is needed and what stain should I use? – K.H.
A: If the underlying wood is still solid, You can certainly stain old redwood siding as long as you do the necessary prep work. As with any paint job, the prep work is the most important part of the job, and often the most time-consuming.
If you stain right over the weathered surface, I don’t think you’ll be very happy with the uneven appearance or with the longevity of the finish. Applying a finish to weathered wood is like painting over chalk on a blackboard. When the chalk falls off, so does the paint.
The same is true for applying paint or stain over dirt. The paint bonds to the dirt, not the underlying wood. For that reason, it’s best to thoroughly clean the surface before staining.
The quickest way is pressure washing, but this is also the most risky as you can gouge out soft areas of wood, leave a lot of raised grain, and force water into the wall system. If you do decide to try pressure washing, make sure that the contractor is experienced with soft wood sidings and uses an appropriate low pressure.
Also do it in dry weather, when the building will have time to dry out thoroughly before applying the new finish. Water can also penetrate to the back of the siding through joints and nail holes, so extra drying time is recommended.
Low-Pressure Sprayer or Sponge
A more sensible, but more labor-intensive, approach is to use a commercial cleaner or mix your own. You can apply the cleaner with a large sponge, but it is quicker with a low-pressure sprayer which you hand pump.
Low-pressure sprayers are commonly sold for gardening and applying deck cleaner and include a wand for spraying a wide band of cleaner with a sweeping motion. Spray on the cleaner, scrub it in with a large, stiff cleaning brush, let it set for several minutes, then rinse with a hose. DO NOT use a metal brush as this can leave metal particles that will stain the wood.
The type of cleaner you need will depend on the conditions. If the siding is a uniform gray, without dark spots from mildew or water stains, then you can use a mild detergent. If you need to lighten sections of the wood, then you will need to use a bleaching agent in those areas.
Choosing A Cleaner
Many commercial cleaners are sold for decking, which are equally suitable for siding. However, avoid strong bleaches and cleaners that can harm the wood and create a health hazard for the user. In particular, avoid products with chlorine bleach or strong alkali cleaners such as sodium hydroxide or potassium hydroxide.
Instead look for the active ingredients oxalic acid, sodium percarbonate, or soda carbonate (also called soda ash or washing soda). Oxalic acid and sodium percarbonate will both lighten the wood to some extent and are useful if the wood has darkened.
Sodium carbonate is more a straight cleaner found in many household cleaning products and detergents. A popular product called Oxyclean is mainly sodium percarbonate (an oxygen bleach) and sodium carbonate (a cleaner)
You can find these compounds in a number of commercial products, or just buy the basic chemicals for a lot less money and mix your own. While these are less harmful than the other cleaners, they can still be harmful to your skin and eyes in concentrated form, so be sure to use gloves and eye protection when mixing any applying.
For troublesome stains, mix a paste of three parts oxalic acid to one part water, and apply the paste with a brush or spackle knife. Let it sit for at least 15 minutes, then rinse off.
For larger sections that need lightening, spray on oxalic acid, scrub it in with a non-metal scrub brush and let it sit for several minutes. Then rinse thoroughly.
Whatever product you use, test it on a small area before applying to a larger section or the whole house.
Once the wood is clean and dry, it’s best to lightly sand the wood with 80 or 100 grit paper using a palm sander or orbital sander. This will remove any loose material and rough up the surface for an improved bond with the new finish. You’ll want to sand enough to expose solid wood to accept the new stain.
Choosing a Stain
If you want the redwood siding to retain the dark red color of freshly oiled redwood, then you should consider choosing a stain with that color pigment. A clear finish will give you that appearance when the first apply it, but it won’t last more than a couple of years. And oil finishes promote mildew growth unless they contain effect mildewcides.
To keep a “natural” reddish look, most people choose a stain that is roughly the color of freshly oiled redwood. Definitely try a sample before committing to the whole house. In essence, you are tinting the redwood to color of redwood. This will help it retain the original color rather than weather to gray again.
For most situations, I prefer semi-transparent oil-based stains. These have enough pigment to add some color, which retaining the natural look of the wood. You will need one with a mildewcide or you can add this to the stain yourself since oil promotes mildew growth in redwood. You may be able to find a finish formulated for use with redwood.
Oil vs. Water-based Stains
In many states, including California, it is increasingly difficult to find any oil-based finishes due to environmental laws. There are water-based finishes and hybrids, often sold as “oil-based finishes that clean up with water.” Read the label carefully to find out what you are buying.
One-hundred percent acrylic (water-based) stains are generally available in semi-solid and solid formulations. These are somewhere between a stain and a paint, with a coating thickness between the two. On the plus side, solid and semi-solid stains show more of the wood texture than a paint, tend not to peel like paint, and are easier to refinish. On the downside, they won’t last as long as paint.
There are too many products on the market, which constantly change their formulations, to make a specific recommendation. Also a product with the same name may have different ingredients in different states due to environmental laws. That said, some product lines that I have used with good results on wood siding are Cabot, Penofin, Sikkens, Flood, and Messmer’s. Just make sure you read the label! ‘
If you need to renail areas, go with stainless-steel fasteners if you want to avoid black stains around the nails. — Steve Bliss, BuildingAdvisor.com
Read about Finishing Wood Decking