Using primer to prevent stains
What is primer?
In woodworking, a primer is a preparatory coating applied to wood surfaces before painting or staining. Its primary purpose is to enhance adhesion, seal the surface, and improve the durability and appearance of the final finish.
When it comes to blocking tannins on wood, specifically in the case of using certain types of wood like cedar or pine, a primer serves an additional important function.
For more about tannins, click here.
A primer used for blocking tannins typically contains specific chemicals that seal the surface of the wood and prevent the tannins from migrating through the paint or stain. These primers are often labeled as “stain-blocking” or “tannin-blocking” primers. They create a barrier between the wood and the finishing product, ensuring that the tannins do not affect the final appearance.
How to stop tannins from bleeding through
- Ensure that you are working in a well ventilated space and are using tarps to protect the surface around the piece you’re working on.
- Sand new pieces of wood to prepare it for primer.
- Sanding is going to make the wood grains even and more receptive to the stain-blocking primer. Use 120-150 grit sandpaper and rub it back and forth over the surface of the wood until it is smooth to the touch.
- Remove sanding dust from the wood with a water-dampened rag.
- Getting all the dust off the wood before you prime is essential to having a smooth painted surface.
- Use a damp rag to wipe off dust and allow the wood to dry completely before moving on.
- Apply oil-based stain-blocking primer to combat tannins and water stains
- An oil-based primer does have a strong smell and requires paint thinner for cleanup, but it is the most reliable in blocking stains from knotty wood, water, or wood that naturally stains (like redwood, cedar, and mahogany).
- An oil-based primer is also helpful in eliminating strong odours from pieces of wood.
- After stirring, use a paintbrush to spread the primer over the wood in long, even strokes.
- Oil-based primer is thick, so you will need to use a little force while painting to get it to spread evenly.
- Work as quickly as you can, as the primer gets tacky as it starts to dry and will show brush strokes more easily.
I typically will use a chip brush or foam roller to apply. That way when I’m done, I don’t have to worry about cleaning them!
- Let the primer dry completely before sanding smooth or adding any layers of paint.
- Depending on the humidity where you are working, it may take several days for this to happen. The wood should be dry to the touch once it is finished; if it feels sticky or if your fingers drag when you run your hand over it, it isn’t dry yet. It’s best to follow the instructions on the products that you’re using.
References:
https://logic-bespoke.com/timber-leaching-and-tannins
https://www.aquacoat.com/blogs/news/how-to-prevent-tannin-bleed
https://www.fs.usda.gov/wildflowers/ethnobotany/tannins
https://www.woodmagazine.com/wood-supplies/lumber/avoid-acidic-woods-to-protect-a-precious-flag
https://en.wikipedia.org/wiki/Tannin
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