How to Stop Sap From Coming Out of Wood

One of the most frustrating problems a DIYer or woodworker can encounter is a “finished” project that begins oozing sap. When this sticky substance appears on the surface of a project, it is a bear to clean up. On top of that, it requires additional time and effort to complete an already “finished” project.

We have tips and tricks for preventing sap from appearing on your wood. I will also offer advice on how to avoid choosing potentially sappy wood on your project.

Stopping Sap from Coming Out of Wood

A basic grasp on what sap is and how it works will help me understand why it is leaking from my wood. Sap is an essential element of a healthy tree but a great annoyance to woodworkers.

Understanding Sap

Wood sap is a complicated term referring to the highly viscous liquids that flow within trees to transport energy throughout the tree. There are actually two different liquids flowing within most trees: xylem and phloem.

Sap is often colloquially referred to as tree blood. Like blood in the human body, sap is part of the circulatory system of a tree. Cuts to the tree’s skin or bark are also filled by sap to prevent infection.

Like the human body where oxygen-rich blood is pumped out from the heart and blood in need of oxygenation is pumped back into the heart, trees have a two-fold sap system. 

• Xylem conducts water and minerals absorbed by the root system up the trunk out into branches.
• Phloem contains and moves energy produced by photosynthesis from the leaves into the branches, trunk, and roots. Phloem is the sticky, sweet stuff I get from tapped maple trees.

When a tree is felled, sap still exists within the tree’s nutrient transport system. Removing sap efficiently is time and resource intensive. Sap can be removed before or after wood is cut into boards.

Kiln Dry Wood to Remove Sap

If timber mills want to reduce the amount of sap in wood, they must take intentional steps. These extra steps cost money and time which is why buying sap-free lumber is more expensive.

You could mill your own wood using a portable sawmill, or by using a bandsaw mill, but that is unpractical for most people.

At your local lumber yard or home improvement store, look for kiln dried lumber. One way to stop sap from coming out of wood is to kiln dry your wood before using it in your project. You can save money by drying your own wood.

If you have green lumber with high moisture content, you can create your own kiln dryer with relative ease. Most of the materials to build the kiln dryer are inexpensive as well.

The goal is to create an airtight, plastic surround in which the wood will dry. Inside the plastic surround will be a dehumidifier, a fan on one end to draw air into the surround, and a vent on the opposite end to expel moisture.

The wood will rest on a frame with small, wooder spacers separating each layer of wood as it dries.

Tools and Materials:

• Miter saw
• Jig saw
• Screws
• Dehumidifier
• Plastic sheeting
• Seam or duct tape
• Box fan
• 2” x 2” wooden spacers cut to length
• 2” x 4” lumber to create a frame

Method:

1. Lay down plastic sheeting. 
2. Create a frame from 2” x 4”s on top of the plastic. The frame should be built of 2x4s on edge. Using a miter saw, cut two 2x4s to length that on edge are as long as the timbers to be dried. 
3. Use a jigsaw to notch cross beams from 2” x 4”s on edge to serve as connections between the long outer frame pieces.
4. Screw the notched connector boards to the frame. Run cross beams every two feet.
5. Lay your wood to be kiln dried on the frame you have just created. Leave at least two inches between each board. 
6. After putting down a layer, place 2” x 2” spacers perpendicular to your drying lumber, at least every two feet.
7. Lay down another layer of lumber spaced at least two inches apart.
8. Place more spacers, followed by another layer of lumber until all of your wood to be kiln dried is stacked.
9. Tape sheets of clear, plastic sheeting together until you have enough sheeting to cover your stack of wood.
10. Place your humidifier on one end of the frame and the box fan on the opposite end, along with any necessary electrical connection to power both dehumidifier and fan.
11. Drape plastic over the stack of wood.
12. Tape the draped plastic to the plastic beneath your stack of wood. Tape around your box fan, but enclose your dehumidifier. If your dehumidifier has a drainage tube, run that tube out of the plastic surround, then seal around the tube.
13. Turn on your box fan.
14. With your plastic surround inflated by air circulated by the box fan, cut an air outlet on the end  of the surround opposite the box fan above the dehumidifier.
15. A small cut in the plastic can be made to turn your dehumidifier, then seal the cut with seam tape.
16. Continue using this kiln to dry the wood until moisture is brought down to around 9%.
17. Moisture content of the wood can be checked through the dehumidifier access.

Stopping Oozing Sap in Finished Projects

If your project is already completed and it is too late to kiln dry your wood, you need to bring down the moisture content of the wood in your completed project.

As wood dries or is exposed to vast temperature variations or moves from moist to dry climates, cellular pressures change. Changes in pressure causes sap to be pushed from wood at the board’s weakest locations. In most lumber, the weakest places are around knotholes.

Chances are, if you have sap oozing from a finished project, it is coming from a knothole. If there are no knotholes, sap also can be secreted between board seams.

If you remember my analogy between blood and sap, the quickest cure for oozing sap is to cauterize the wound. This option works best on oozing pieces that have yet to be painted or sealed.

If your wood piece has already been sealed or painted, applying the heat gun to oozing areas will require the piece to be resealed or repainted.

When sap reaches a temperature above 170 degrees Fahrenheit, the sugars will crystallize. The sap turns from liquid to crystal and will no longer ooze.

Tools and Materials:

• Isopropyl alcohol
• Paper towel
• Top rated heat gun
• Welder’s blanket
• Gloves

Method:

1. Place the welder’s blanket beneath the oozing piece.
2. Put on gloves
3. Scrub the exposed sap with a paper towel dampened with isopropyl alcohol. The alcohol will break down the carbohydrates within the sap and allow it to be easily wiped up.
4. Completely dry the wood using paper towels.
5. Turn on the heat gun.
6. Apply heat to the oozing areas while moving the heat gun side to side. Never linger with heat in one spot, otherwise you risk scorching the surface of the wood.
7. You should see and possibly hear sap crystallizing water evaporates from the sap near the surface of the wood. Sap may also bubble to the surface and immediately harden.
8. After the areas oozing sap are cauterized, scrape away melted sap.
9. Reseal or repaint your piece. A layer of shellac may help seal sappy wood better than polyurethane.

Conclusion

Wood that is oozing sap can be addressed before your project begins or after it is completed. Kiln-dried wood with its low moisture content is the surest way to prevent sap from coming out of your wood. Finished projects with sap coming out can also be spot treated using a heat gun to crystallize the sap.