Finishing Koa KamiPLY or WashiPLY Veneers and Lumber
Hawaiian Koa is a great wood to apply a finish (also called varnish). It loves to accept pretty much any kind of finish, from all different kinds of oils, waxes, urethanes, lacquers and nitrocellulose. While I am not particularly a finish expert, I have worked with so many woodworkers, and gotten all kinds of feedback over the last 30+ years of being in the Koa wood business. We also manufactured furniture and ukuleles for over 10 years so I can only speak directly to our own experience. But I do want to re-iterate, Hawaiian Koa LOVES to have a finish applied. That's where the beauty of the natural wood really comes through.
Also, Koa's woodworking properties are very similar to Walnut wood, so searching the internet for finish techniques on walnut wood will also apply to Koa, and there is SO much info when it comes to working with Walnut wood.
So, now that we know Koa loves to have a finish applied, the real question to ask yourself is how much work I want to do and what the final resulting sheen I want to achieve and what level of durability do I need.
NOTE: our KamiPLY and WashiPLY veneers are cut so fine very little sanding is needed. Be careful not to sand too hard or you will "burn" through the wood to the paper. Another thing to take note, is the paper we use, Polybak by Richwood Industries, is pre-sanded on the back for better adhesion to your substrate.
A very easy method, without the need for any electrical tools, is the tried and true, oil + wax finish. While this method is easy, it doesn't necessarily allow Koa to "pop" likes it wants to. This is especially true if the Koa is figured or curly. It is considered a matte finish and generally all matte finishes will decrease the level of curl visible. Please check out this link for some info on the grades of curl of Koa.
For example, if the Koa you want to finish is a "medium" or 3A curl, using the oil-wax method will not allow much of the curls to be visible when finished. Yet, if an oil-based finish is used, the curly wood becomes more visible. We had some super curly Koa wood ukuleles that when they were finished the curl wasn't all that visible which was kind of sad to be honest. But many people prefer the subdued tones of a matte finished product.
Durability - your finish type will also affect the final durability of your product. Oil based finishes are generally most durable and definitely more than a wax finish. A wax finish leaves the raw wood kind of exposed so for an instrument or table, if you accidentally bang it, the chance of the wood denting is highly probable. Whereas a glossy, oil-based urethane or lacquer, puts a protective coating over the wood and less chance of having the wood being dented.
For that reason, urethanes or lacquers are much more preferred for a veneered product. Since there is so very little of actual Koa wood in the veneer, it is recommended to go for a stronger finish. But of course it also depends on what the product is and where it will be.
Pore-fillers - When we were making ukuleles, we often added a pore-filler, which is more needed for lumber than for veneers. But do be careful on which pore filler, because they can sometimes change the color of Koa to give it a kind of greenish tint. We generally used a mahogany or rosewood colored pore filler which matches Koa well. Some of the clears gave it the green tint, although it could have also been affected by our water-based finish we were using.
That's where it takes a LOT of trial and error to really get the final look you want.
Water-based finishes - while they claim to be more "earth-friendly" they really are not because of how they are made (that's is a whole different blog topic), but they are a great option when working in the home or garage where disposal of the finish is much simpler and won't usually break any laws when disposing it in your sink. PLEASE be sure to check with your local regulations regarding the disposal of finishes. Each area has their own rules and methods. I only know about Honolulu county issues working in a professionally-setup spray-booth and using the disposal service to rid ourselves of any leftover chemicals.
I do want to give a great shout-out here to Makoto Harada of Honua Ukuleles (https://honuaukulele.com/). He has spent the last 10 years or so trying to perfect the water-based finish system and his ukuleles are amazing. When he was working with us, we do so many trial and errors and the various mixes to achieve the best gloss and truest colors. It was a huge challenge indeed. We also found the water-based finish took a LOT more time to cure and sometimes even months. It remained quite soft for some time and even after letting them cure for a couple of weeks, there were times when the finish picked up indentations from hanging on the ukulele hanger. But for the hobbyist who has the time to wait, they are a great finish to work with.
I would also recommend a shellac base coat especially when applying a water-based top coat. That was one way for us to help give the Koa the initial "pop" it wants to show. We used a wipe-on shellac (don't remember the brand, but most are pretty good no matter which is used).
Oil-based finishes - those are probably the most common and easiest to purchase in any quantities. They will come in any form you want - an aerosol can, a wipe on version or even for the air pressure guns. Generally for our use, we used the more expensive urethane finishes where we mixed our own which gave it a day shelf-life, since we were going for the highest quality of finish and considering it was almost always on Koa wood, it was most fitting. We used ML Campbell Krystal for all our Koa woodworking.
Koa is one of those woods that completely "pop" and change from a 2D to a 3D and even sometimes 4D when the curly is so crazy that it's almost like is been injected with plutonium.
I would check out this link for a lot of good info on kinds of finishes, techniques, and so much more that I know. https://wood-veneer-factory-outlet.com/staining-and-finishing-wood-veneers-for-dummies/
Mahalo for your time and we hope you completely enjoy working with Koa, the most beautiful wood in the world!