Finish for wood that may "move"

[Mike Holbrook]

Finishing a "live edge" slab bar top. I have been studying "Understanding Wood Finishing" by Bob Flexner. I have been searching the index for information on which types of finish work best to control cracking & warping, which concerns me with a 18" x 2" x 83" slab board, not finding the answer. Bob addresses cracking and warping (pg 1-4) as helping slow, but not prevent these issues. Bob seems to prefer the harder finishes like Varnish. I am wondering if a hard finish might not "move" as well as a more flexible one on a board this wide & thick? I have been thinking about using a varnish & oil combo for a few coats and then a marine spar varnish? I have read a post or two where people use to do this but stopped doing it for some reason that was not explained. The bar top is outside on a covered porch, where sun and rain may hit it, but not regularly or for extended periods.

[John TenEyck]

Nearly any wood finish will handle the expansion/contraction of your slab; it's only 18" wide, thickness is irrelevant to the consideration. No finish is going to prevent cracking or warping if the wood is determined to do so. All finish can do is slow down the exchange of moisture into/out of the wood, and that does dampen the tendency of wood to crack and warp.

Spar varnish is more flexible than the indoor variety so that it can tolerate the greater expansion/contraction that is likely to occur with wood outdoors. It also has UV stabilizers in it to retard UV degradation. But to be flexible it is relatively soft compared to indoor varnish; that's probably why Flexner would not recommend it for indoor work. But since your bar top is going to outside with the possibility of sun and rain hitting it, it would be a better choice than an interior varnish.

For what it's worth, I just built a table for my deck. It's made of Sapele and I finished it with Penofin Oil for Hardwood rather than use a film finish. I'll have to clean and re-oil it every year, maybe more than once, but the finish will never crack or peel. On the other hand, the oil doesn't show off the grain as well as varnish and the wood could get stained if something that has a tendency to do that is left on it too long. Pick your poison; there is no perfect outdoor finish.

John

[Mike Holbrook]

John, the information you supply regarding, I think you meant marine spar varnish, suggests it is more flexible, which is why I understand it is used on wood boats and yachts that get wet and stay that way. I believe the flexible coating does not flake off, split...due to it's flexibility. I think it is able to move with the wood vs a harder finish that might crack.

The Penofin sounds interesting. I could not find what it is actually made from though. The information mentions that the product contains Brazilian Rosewood Oil. I have read that "oils" are not the most durable substances so I am wondering if this "oil" is actually a mix of Varnish & oil or some substances that combine flexibility and hardness?

[John TenEyck]

Hi Mike. My understanding is that all spar varnishes are more flexible than interior varnish, but I certainly could be mistaken. For sure marine spar varnish is, as you noted. I've used a fair bit of Epifanes Marine Varnish, and it stays almost sticky for a long time, but it survives a long time, too.

I'm not sure what's in Penofin Hardwood Oil. Brazilian Rosewood oil sounds like marketing fluff to me. For sure it has UV absorbing oxide pigments. I'm pretty sure, however, there is no varnish in it. It will not form a cured film. If you leave any on the wood it ends up being a sticky mess. You put it on, let it soak in, and wipe off what doesn't absorb after 5 mins. I plan to keep a cover on my table, but still expect I will have to clean and re-oil the table every year, like I do my deck. Outdoor wood is a pain to maintain with a natural finish.

John

[Mike Holbrook]

John I believe Marine Spar Varnish just adds UV protection to Spar Varnish.

I plan to use either a Marine Spar Varnish or a Varnish, probably with UV protection, as the outside coat. I am trying to figure out whether to use: "long varnish" , oil, oil varnish mix... on my board before doing final coats. I understand that long varnish, oil and oil varnish mixes, due to their extended set up time, have more time to penetrate deeper into the wood. Like you say more oil or oil varnish deeper in the wood may prevent moisture from seeping in and the various results from the ensuing expansion of the wood.

I am particularly interested in oil or oil varnish mixes that may bond well to whatever product I use as the outside coat. I am aware that not all of these products bind together well. I am also aware that using two products can cause cracking or peeling of the top layer.

[John TenEyck]

Mike, I would not mix products. If you look at the directions for Epifanes, for example, you'll see that you have to substantially thin the first few coats. That is done so that it will penetrate into the wood so that the subsequent coats that are applied full strength bond properly. You end up with a film made from one product well bonded to the underlying wood.

I recommend you stick with one product. Don't try reinventing a solution to a problem that has already been solved.

John