You bring up a very interesting issue with wood finishes for preserving green furniture. Wood finishes break down into two categories:
- newer urethane finishes that basically are a strong layer of plastic that sits on the wood, and
- traditional oil and wax-based finishes that sink into the wood.
Oil and wax-based finishes tend to need renewal and replenishment. People started using urethane finishes because they are waterproof and low-maintenance, which would be a priority for a highchair that is wiped down time after time.
However, there are pros and cons with each option.
On the plus side, the polyurethane finish is waterproof, and the water-based formulations can be low-odor and low-VOC. One obviously needs to wipe down a highchair frequently -- I have seen the rice cereal fly! This coating would be the most "sponge-friendly."
- The AFM Safecoat product line has three clear finish options. They are a paint and finishes company that originated to provide non toxic finish and coatings for those who were concerned about or sensitive to chemicals.
- Another choice would be the Vermont Natural Coatings Poly Whey product. This is an interesting product because it uses whey proteins as the bonding agent, thus using less petroleum-based plastic. The whey is a renewable byproduct of the Vermont cheese industry. The product seems to have gotten good reviews from woodworkers thus far, although it is a fairly new company. I am eager to try their products myself!
On the minus side, polyurethane finishes are not traditional, and for that reason should never be used to refinish heirloom-quality antiques. Also, because the plastic coating is impervious and inflexible, it does not allow you to replenish the oils in the wood. Wood does tend to expand and contract, and polyurethane can crack, especially along seams. If this occurs, it is possible that a child might end up consuming a few flakes of the product.
And finally, when it comes time to refinish the piece again, the whole piece would need to be done -- you cannot touch up urethanes without it showing.
Natural oil or wax-based coatings
The natural oil or wax-based coatings feed the wood and give it a more natural finish. The grain tends to show more than with urethane finishes. These finishes both coat and sink into the wood, but they are not impervious. I guess you could say they would be water repellent rather than waterproof.
Since this kind of finish is used on wooden cutting boards or wooden food-prep surfaces, it is food safe.
- Osmo, which makes natural floor finishes, also has two products that are good for furniture: the Wood Wax finish, and the Top Oil.
- Another option is a pure oil finish, such as natural Tung oil.
These finishes require multiple coats to be durable, but are extremely easy to touch up. If the item gets scratched, just apply another coat of the oil and it will fill in the scratch. Re-coating also helps maintain the finish.
Traditional wood finishers would also include lacquer and shellac as good options for furniture. Both of these are solvent-based, and thus not low-VOC. I would not use these in my home unless I could leave the item in a well-ventilated area for a time before using, to protect indoor air quality.
Ultimately, I think either type of natural finish would work well on your highchair.
- With the oil and wax-based finishes, you are committing to a little more maintenance, but ease in touch-up and a prettier depth of grain.
- With polyurethane, on the other hand, you have the more waterproof finish.
Good luck with your project.
For more information:
Read Piper Kujac's Q&A "Which wood finishes are safe to use on children's furniture?"
And read our backgrounder "Selecting Healthy and Environmentally Sound Clear Wood Finishes" for more detail on urethanes, and natural wax and oil finishes.