Recently I finished editing Clearwater’s maintenance manual. The original is an amazing document that was created in the late 70′s, and has provided invaluable information to bosuns over the years. I highly recommend maintenance manuals and logs to everyone out there maintaining any kind of boat. Even if it’s just you and your own little dinghy, you’d be amazed at the little things you forget, and how helpful a full and detailed record can be for proper, long-term maintenance.
What follows is an excerpt from one of my additions to the manual. I have cared for brightwork here and there over the years, but by no means am I an expert. As I’ve gone from ship to ship, I’ve often been curious and confused about the very specific wood-finishing choices that are vehemently touted by each one. Everyone has their favorite, and their reasons why.
With a manual containing 30 years worth of mixtures and instructions, I was frustrated by the lack of explanations for LTV here, varnish there, and simply Penetrol in other places. I did a little research, and found that there are few places that give fact-based, comprehensive explanations on the differences and advantages of each one. I did my best to compile that research here.
Note, there are many types of finishes not mentioned here. I am only writing about the types used on Clearwater. Also, I have no references! At some point I may write a more in-depth article on this topic that will include other finishes, and…references.
Varnish is made from a combination of resin (either plant-derived or synthetic), drying oils (most often tung or linseed), solvents (turpentine, etc.), UV protectors, and sometimes driers. Traditional knowledge says that varnishes give superior UV protection to wood that gets sun exposure, and a relatively long-lasting, flexible finish. Varnish’s flexibility makes it ideal for moving parts which might cause other harder finishes to crack, allowing moisture to damage the wood.
A proper varnish job requires a somewhat experienced hand, not only for a nice-looking finish, but also to ensure that the finish takes. If done improperly, varnish can crack, peel, delaminate, bubble, or never fully harden. Varnishing also requires a somewhat dust-free environment, nice weather, and lots of time to apply many coats. For our purposes on Clearwater, we tend only to use varnish on exterior wood that has a low likelihood of getting banged or chafed, such as hatch covers, since touch-ups are somewhat challenging and time consuming. Someone who knows what they are doing should scuff and apply a top coat of varnish once per season.
Penetrol is at the opposite end of the spectrum from Varnish. It has no UV protective qualities, and is even rumored to cause greying or darkening of the wood when not removed and reapplied regularly, due to the linseed oil content. Although it keeps wood from drying out by helping to restore its natural oils, it doesn’t last long and needs frequent (almost weekly) applications. It’s nice because it is very quick and easy for anyone to apply, very easy to touch up chafed spots, kicks quickly, and doesn’t leave a tacky residue.
Polyurethane is a finish/sealant that has a very similar application and appearance to varnish, and therefore the two are often used interchangeably. Polyurethane differs from varnish in many ways that don’t matter to us very much, namely the way it is made, and the chemical process it goes through to “kick”. What IS relevant to us is that a polyurethane finish tends to be MUCH harder than varnish, meaning it holds up much better under certain circumstances. The down side is that it also tends to be less flexible, which means that it will crack much more easily on pieces that flex.
A point about UV protection: I’ve always been told that the difference between varnish and poly is that varnish protects against UV damage and poly doesn’t. From what I gather, this isn’t fully true, at least anymore. Both varnish and poly CAN contain UV protective additives, but from what I’ve read, over the long haul, the UV filters in polyurethane break down faster than those in varnish. I have not myself worked on the same piece of half-varnished and half-polyurethaned wood for 10 years, so I can’t speak from personal experience.
There are a few different wood oil recipes (also known to boaters as “boat soup”) that have been used over the years. It is generally a mixture of two to four of the following in varying quantities: varnish, boiled linseed oil, turpentine, Penetrol, and Japan Drier. Boiled linseed oil is a plant-based oil that can help restore wood’s natural oils and keep it from drying out, but blackens wood quickly without frequent applications (more expensive tung oil can be used instead to eliminate this problem). Varnish gives some UV protection and a harder finish, but slows kicking time. Turpentine thins the mixture, allowing it to be absorbed better and dry more quickly. Penetrol is a brushing agent that both thins and contains oils that can be protective. Japan Drier speeds up the drying process, but too much causes discoloration.
“Boat soup” is relatively easy to apply and can give great results, but depending on ratios, you can end up with a tacky finish that never hardens, or a weird color. It also takes some time to dry, and requires regular reapplication. The recipe we’re currently using is listed below, but if it’s giving you trouble, ask some old salt and you’ll get some good advice. Or do some experiments of your own and you can name the new mixture after your favorite Aunt.