What I want to talk about today is wood of course. More specifically I would like to discuss the proper treatment of old wood in prepping for a durable painted finish.
As with all painting tasks, understanding your substrate is the first step in choosing your finishing approach. You should approach metal different than you do wood. That seems obvious. However, even most pros don’t realize that different types of woods also require different approaches. The “type” of wood that we are most concerned with today is old wood. As historic restoration contractors, we work primarily with wood that is at least 80 years old. We have come to realize that old wood must be approached differently than new lumber if you want to have a quality finish.
Before we start talking about what you need to do, let’s delve a bit into wood and how it ages. Wood is a fantastically resilient material. It can last essentially forever if a few simple conditions are met. It must be protected from the sun and it must avoid prolonged periods of moisture saturation. Most people think that water rots wood. However, water on its own does not hurt the wood at all. It is in fact fungus that causes all rot and fungus requires a certain amount of moisture in order to survive. If the moisture content in the wood is below 20% the majority of the time, with the ability to dry out when it rises above that, then the wood will never rot because the fungus does not have a chance to grow.
Quick tangent: I always find it funny when we work on windows from the early 1900’s. Up until that time, standard practice in building windows was to assemble the window parts, drill a hole through the tenons, and insert a wooden peg to hold the joint together. At the turn of the century, metal fasteners were becoming cheaper and easier to acquire for builders and they started using steel nails instead of wooden pegs. Now 100 years later, we are finding something interesting. The metal nails are failing. They are often completely or at least mostly corroded yet the wood surrounding them is still perfectly sound. The moisture content in the wood was harmless to the wood but the “mighty” iron crumbled under it, literally. All of those fantastic homes from the colonial period wouldn’t still be standing if they were held together with steel nails. Food for thought.
So we know we need to keep the wood from getting wet for prolonged periods but there is another enemy to deal with: the sun. The sun, and it’s UV rays in particular, will also damage wood. The lignin in the wood cells is damaged by UV light in a process called photo-oxidation. Lignin is what gives the cell walls in wood it’s strength. As the lignin in the cells degrades, cracks develop in the wood. These cracks allow surface water a place to get in. In addition, since the cell walls are compromised they are now more absorbent. Now the surface of the wood starts absorbing water like it is the end grain. Moisture content in the wood will begin to rise and the wood is now more amenable to fungus growth.
Clearly, the takeaway is that you want to protect your wood from the UV rays of the sun and high levels of moisture content. The obvious choice here is paint. There are a number of clear finishes that can perform well but nothing can compare to paint. A quality paint job will block almost all UV rays and keep the moisture from getting into the wood.
So the question now isn’t if we should paint the wood but how do we keep the paint from failing. We can get into the details of a quality paint job in another post where we discuss paint prep, primers, and paints. Today we will just focus on one particular step that is often required when dealing with old wood that is not required with new wood. After 100 years, it is safe to assume that the wood was not always perfectly protected from the elements. It is highly likely that various sections spent some time unprotected or poorly protected from the elements. Thus, there has probably been at least some UV damage to the wood. If you do not address this you may run into problems.
As was mentioned earlier, as the lignin in the wood cells is degraded it becomes more absorbent and sponge-like. This presents a problem when it comes to priming and painting the damaged wood. New wood will very lightly absorb the primer, which is great for getting good adhesion, but it will mostly sit on the surface to cure. The UV damaged wood however will try to absorb every last bit of liquid it can until the damaged wood cells are full or the liquid runs out. The problem is that primers are a delicate mix of solids and liquids. The liquids are there essentially to help the solids assemble and cure properly. All paints recommend you to not paint when temperatures are too high or in direct sunlight. That is because if they dry too quickly the paint or primer will not have enough time to cure properly. The same thing is happening when you paint the old damaged wood. The wood will suck up all of the liquids and leave all of the solids on the surface. It is like trying to use a colander as a soup bowl.
If anyone has ever had a section of wood on their home (usually in a high sun exposure location)that they just cannot keep the paint from peeling on no matter how good they prep it, this is probably what is happening to you. The biggest problem is that you really can’t tell if wood was damaged or not by looking at it. Nevertheless there is a simple way to deal with it. There is a pre-treatment that you must do to the wood prior to priming. What we use on windows in our shop and we call our “sealer” is really a consolidating oil-resin that we make ourselves. Historically, painters would use a 50/50 mix of linseed oil and turpentine as their recipe and that still works fine. Since there are some concerns that recipe may be a bit susceptible to fungus growth, we use a 50/50 mix of mineral spirits and an alkyd oil instead. The oil we use is usually the product know as Penetrol, which is sold as an oil paint additive and conditioner. We simply brush this sealer onto the bare wood. This is when you will find out if the wood was damaged. If it was damaged you will see the wood absorb the sealer as soon as you brush it. If it wasn’t damaged you will see it sit on the surface of the wood. When you encounter sections that are sucking up the sealer, just keep brushing it on until it stops absorbing it (or until you give up). After you give the sealer a bit of time to dry, you can move to priming and painting as you would any other wood. It’s that simple.
If you perform this one simple step every time you encounter some old wood that may have been damaged by UV rays you could avoid a lot of aggravation and considerably extend the life of your paint job. The devil is in the details. Sadly, this is a detail that even most high class pros aren’t aware of but now you are!