Rick White: It all comes down to strength. The biscuit joint doesn’t give you the strength you’ll get with a mortise and tenon. One issue to take into consideration is how the piece is designed and where the stress points will be. As a general rule, however, if I’m building furniture, I use the mortise and tenon. Furniture gets a lot of stress and wear and needs to hold together. For cabinetry, I’ll use a biscuit joint. Most cabinets just hang on the wall and don’t see a lot of stress, so biscuits are usually strong enough.

Ian Kirby: The larger question asks about guidelines for making decisions about the strength of joints.

What we do is make our judgment based on empirical values; we rely on experience and observation. The outcome is that we usually make the joint overly strong. We have no idea by how much it’s overbuilt, but as long as it doesn’t break during its useful life, that’s OK. Mortise and tenon type joints are the ones we use to join parts together to make structures that have to withstand stress ? chairs, beds, tables and the like. Maximizing joint strength seems to me to be a good idea – knowing more about it was what caused me to put forward a proposal for a dissertation whilst studying for my Wood Science and Technology degree. It’s now more years ago than I care to think about, but loosely said, it was a comparison of the strength of mortise and tenon joints and dowel joints.

Well, the proposal got nixed because it was determined to be “insufficiently academic.” Instead I did research on some microscopic comparison between two rare species of softwood – a subject of interest to me and two other people in the world. A couple of years later, FIRA (The Furniture Industry Research Association), a British organization, decided to fund research into what amounted to the same study I had proposed for my first dissertation.

The research was funded for one year – no report. Then it was funded for a second year – no report. Then a third year; the result was “there are altogether too many variables for the data to be of any practical value to a woodworker.” Here is a sampling of the variables:

– How rough or smooth are the walls of the tenon, the mortise and the dowel hole?
– How round is the hole?
– How round is the dowel?
– How tight or loose are the interfaces of the parts?
– What glue was used?
– How thoroughly were the parts wetted with the glue?
– What species of wood was used?
– What was its growth rate?
– Were the joint parts tangential or radial tissue?
– What was the moisture content of the wood?
– And so it remains.

In a mortise and tenon type joint, there are two things to take into account with regards to strength: the mechanical strength and the glue line strength. To get a gauge of the mechanical strength, put the pieces together dry and test the sort of stress it would take to cause them to come apart. In other words, how much does the joint rely on the glue in order to stay together?

To illustrate the point, I’ll briefly describe four of the options you generally have with this sort of joint situation: a traditional MandT, a loose tenon, a dowel joint and a biscuit joint.

Of the four, the traditional M and T is the only two-part joint. The others all bring into play a third element. Since the sizes and sections of the parts vary, each mortise and tenon is designed to be as strength-balanced as possible. The tenon part is no stronger than the mortise part. It’s also designed to have the maximum glue area possible.

A loose tenon has much the same strength as a mortise and tenon. What you begin to realize now is that form doesn’t follow function ? form follows economy. You only need one machine to make a loose tenon: a slot mortiser. The question is: can you afford one?

In spite of all the theoretical disadvantages and failings of a dowel joint, it mostly works.

The biscuit joint has quite a bit less mechanical strength than the other three and its glue area isn’t great. But it scores big on the accuracy of its glue line. The walls of the joint are smooth, the plate swells to form a tight interface and, in many cases, it’s possible to double up the biscuits so the gluing area is doubled.

At the end of the day, whatever you decide, you must ask: Is the joint sufficiently strong to outlast the useful life of the piece?

Rob Johnstone: Mortise and tenon joints can be decorative, but the main reason to use them is to join two pieces of wood in a strong efficient manner. My opinion, and I know it is in the minority, is that biscuit joints are handy for accurately aligning joints and nothing else. I don’t believe an edge that is joined for glue-up is any stronger for having biscuits in it, but it is easier to line the edges of the glue joint up in a hurry. And if you are edge gluing five or six boards, biscuits are a real time-saver. So if the mortise and tenon joint is inteded to aid in alignment of a piece rather than creating a strong joint, I say go ahead and biscuit away. If you’re asking if the biscuit joint would be stronger or superior in any other way, I say no.

Ellis Walentine: Yes and no, depending on the type and severity of the stresses you expect. Technically, a biscuit joint is a type of mortise-and-tenon joint ? where one piece of wood penetrates and is glued into another. Both types of joints keep parts aligned and offer resistance to twisting, shearing, levering and pullout forces on the assembly. The strength of every type of mortise-and-tenon joint depends on several things.

1. The type of wood determines the mechanical strength of the joint: harder, stiffer woods are more resistant to breaking.

2. The design of the joint is also important. It involves the proportions of the mortise, the tenon and the “shoulders” where the two parts actually meet.

3. But, for practical purposes, the glue area is what really holds the joint together. Biscuits don’t offer as much glue area as traditional tenons, so you need more biscuits to equal the pullout and levering strength of a tenon. On the other hand, biscuits generally provide enough glue surface to keep the shoulders tight, which makes the joint just as solid. Biscuits are best suited for plywood case construction, for which they were originally designed. They also are fine for long edge joints, some cabinet doors and other types of frames, including those with mitered corners. Properly designed biscuit joints ? often involving multiple, ganged or stacked biscuits – have also been used for demanding tasks such as full-size doors.

Personally, I prefer the security of deep, accurately machined tenons for heavier-duty applications where greater mechanical strength may be needed.

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Chris Marshall: Always nice to have tool options at your disposal! Regarding cutting those plywood circles, if I was making just one Lazy Susan, I’d probably use a plunge router mounted on a circle-cutting jig. Drill a small hole partway into the bottom face of the plywood (where it won’t show anyway), set the circle-cutting jig to the radius of circle you want, and then cut the circle round in a series of several deepening passes with the jig’s centerpoint fixed in the pilot hole. Either a straight bit or an upcut spiral bit will do this job nicely, but be sure to use sharp bits to minimize splintering when the bit passes through to the other “show” face of the plywood.

You can buy circle-cutting jigs, or make them. For one plan, see Sandor Nagyszalanczy’s jig saw circle cutting jig.

Alternately, you could cut these circles on a band saw using a circle-cutting jig made for that machine. A band saw with a sharp, fine-tooth blade will provide clean edges, and it’s easy to find plans for a band saw circle-cutting jig online. Still a third method would be to cut a round template from 1/4- or 1/2-in. MDF or Baltic birch plywood. Then cut your plywood lazy Susan tops slightly oversize, secure the template to them with a few pieces of double-sided tape, and use a pattern or flush-trim bit in your handheld or table-mounted router to trim them to final size. This last method is probably the most “goofproof” and could also provide you with the cleanest edges for banding with wood or veneer. If I were going to make lots of lazy Susans, this last option would be my pick. It’s fast and simple.

Tim Inman: Depending upon the size of the circles, I would suggest either a band saw jig or one for the router. I use my band saw jig for smaller circles (up to about 24-inches dia.). I use my router for anything larger. The band saw leaves a “sawed” edge where the router is more perfect. But, the band saw is so much faster and easier. I can’t give the whole set of instructions for constructing the jigs here, but the basics are the same for either tool.

My band saw circle jig is a false table that includes a sliding bar embedded into it. This bar has a spike on one end that acts as the centering point for the circles. I can slide the bar in and out to adjust for the radius of the circle I want to cut. You can find plans for these quite easily. The router jig is similar, except the bar for the router jig is fastened to the router. The point or center spike is on the far end. Visualize a trammel bar compass and you’ll have the idea.

I use the band saw jig to cut circles for the bottoms of turned bowls and cups all the time. It makes quite accurate circles, FYI.

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Rob Johnstone: While there is no rule of thumb that I am aware of for reveals in this case, I am with you in thinking that a one-quarter inch gap all around your door is a bit excessive. One truth can’t be denied: you can always make a door smaller, but once it is made, it’s hard to make it bigger. I would make your door with about 1/8-inch reveal on all four sides. Put it in the opening and see how it looks. If you want more gap … you can make the door smaller — if not, you are ready to mount the hinges.

Tim Inman: One-quarter inch OVERALL would leave you with a one-eighth inch gap all around. That’s about right. If you make the gap much bigger, it looks like sloppy work. If you make it any smaller, you’ll end up with binding doors. I despise doors and drawers that bind and drag. There’s no excuse for that in my book! Remember that, over time, your cabinet is likely to shift and go out of square. That little bit extra right now will provide a cushion or tolerance for imperfection later on – and will make your cabinet appreciated in the decades to come.

Chris Marshall: I always wonder what sort of wood other woodworkers are using when they advocate making solid-wood doors and drawers so tight as to barely leave a reveal. Evidently, they aren’t working with the wood I use — the kind that expands and contracts across the grain. Sure, those tight reveals look fantastic during the winter when the wood has contracted, but boy will they cause problems in the middle of a muggy July! I agree with Rob and Tim: aim for 1/8-in. reveals, and you won’t be disappointed. But, if you are making frame-and-panel doors where the panel “floats” inside the rails and stiles, you can get by with a tighter reveal. The doors won’t expand and contract to any measurable degree.

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– Granville Jones

Trying to seal liquid sap is an exercise in futility. Spar urethane (a misnomer, by the way) is probably exterior urethane, and while it would undoubtedly cure over the sap pockets, do you really want active, oozing sap under your cured finish? That can’t end well.

The traditional material for sealing sappy knots is called “knotting” and is made of thick shellac. It works moderately well for a little while but ultimately fails. I’ve seen sap make its way through just about every clear wood finish, thick paint and even through vinyl “contact paper” drawer liner material. There is a way to “set” the sap by heating the wood past the point where sap crystallizes, but it’s probably not practical on your thick pine slab. Personally, I would consider using something else for a coffee tabletop. After all, even the best finish has its limitations.

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There is a product called “amalgamator” meant to do just that: melt lacquer back to smoothness. The problem is that it consists of a mixture of strong primary solvents, and while it will soften and blend a finish, it will take it completely off just as easily. Because it is so tricky to use, the one company I know of that is selling it tries to restrict its sale to professional finishers. Fortunately, there are easier reliable methods.

If the lacquer is thick enough, you can buff out scuffs with either rubbing and polishing compound, for gloss lacquer, or with 0000 steel wool and paste wax, for satin finishes. With lacquer that’s not thick enough to buff, you can clean the surface, sand lightly to blend out the scuffs, then go over it with another coat of the same type of lacquer.

Nitrocellulose and most acrylic lacquers re-melt themselves with each coat. That means a wet coat will soften and blend into the old lacquer, effectively flowing out and hiding the scuffs. Many repair folks, in cases like this, pre-spray the finish with a thin coat of retarder, or even just lacquer thinner, prior to spraying the coat of lacquer. This will help soften and flow the surface, making it more likely to hide unsightly scuff marks.

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Do you have a recommended finishing method for marquetry pictures, given that they typically incorporate a variety of wood types, wood grains, grain directions, etc.? Would you have any pro or con comments about the idea of finishing a marquetry picture with several coats (eight to ten) of Zinsser SealCoat Sanding Sealer, sanding between every two or three coats? I find that this product goes on rather easily and, since a marquetry picture doesn’t experience any appreciable wear, I thought it would be an acceptable alternative to the much “smellier” Deft® lacquer that I’ve been using for a number of years now.

— Robert Swanson
Wichita, Kansas

You chose wisely. Zinsser SealCoat Universal Sanding Sealer is pure, dewaxed shellac, and that is an excellent finish for a marquetry picture. Dewaxed shellac seals well over all woods, comes in a variety of hues, and has good wetting and clarity. I like to flood on the first coat liberally, then wipe it all off. Woods prone to absorb more finish will do so, resulting in very uniform sealing. Thus, by the time you get to the second coat, you have a more uniform surface than you started with.

Because it contains a polar solvent, the first coat of shellac will raise the grain of wood slightly, leaving it not rough, but furry. I like to knock back the “fur” with a very light scuff using 800-grit sandpaper, taking pains to avoid cutting through to raw wood. Because shellac dissolves itself with each coat, you don’t need to sand after that unless you get dust, dirt, brush marks or spray marks (runs, overspray, etc.) in the finish. As long as it goes on smoothly, there’s no need to sand between coats when using shellac.

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– Marvin Wachs
Joplin, Missouri

One of the great myths of outdoor woodworking is that the “right” finish will add the properties you really want the wood to have. It won’t. Thus, I would have started by choosing a wood with high natural resistance to rot and bugs. Old-growth red cedar fits that category, but the cedar we buy today does not. Whatever you do in terms of finish will not change the essential nature of the wood and will provide only very short-term protection, if that.

I haven’t seen any lab tests confirming it, but I’d suspect that of all the finishes you listed — and all of them are certainly acceptable exterior finishes — I’d guess that burning, either with or without oil, will offer the most protection, as it creates a layer of carbon atop the wood.

Personally, I’d line the inside of the planter with an inert gardening plastic, something made to be in constant contact with water and soil. If you use a plastic liner to isolate the soil from the wood, there’s no reason you can’t substitute pressure-treated wood (yes, there is pressure-treated cedar) for more longevity. And, yes, I see the irony in pressure treating new-growth cedar to make it behave the way old-growth cedar does naturally.

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Tim Inman: Distilled water is a known quantity. “Ordinary” water should work just as well, but why push it? Distilled water is cheap, and if it is working for you, why pinch pennies and risk costing you dollars later on? The pH of the water would be the biggest offender, more than likely. Distilled water is neutral, by definition.

Chris Marshall: I agree with Tim — if it ain’t broke, why fix it? Distilled water worked for your paint solution for the cabin, so it should work just as well on your house. When it comes to the alchemy of finishing — be it paint or otherwise — I tend to go with what has proven to work for me in order to avoid unpleasant surprises.

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Tim Inman: Red dyes are classically light fugitive. It is nearly impossible to get a red that will last. Also, as wood ages, it has a tendency to turn brown. So, one thing coupled with the other yields the results you are seeing. Short of some Draconian efforts to get that red you like — efforts that will ultimately fail, given enough time — I suggest you just learn to enjoy the color you have. This brings to mind a memory: I once refinished a desk for a University of Wisconsin professor. It was a nice job, and I really appreciated the work. As a special “thank you,” I added a nice leather desk set for him when I delivered the piece. There were no windows in his office. The light was just ordinary fluorescent tube lighting. After only a few months, I got a call. When the staff cleaned his desk (this would lead us into another long story about how absent-minded professors need help!) they lifted the desk pad to dust the desk top. Guess what? Under the pad it was still the same red mahogany you show in your first picture. The rest of the desk was browner. They feared the desk pad was bleeding into the wood and causing it to turn red. Not so.

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Tim Inman: Paint pigments are basically ground dirt or glass. Non pigments are dyes. Dyes fade. Some dirt will fade, too, but not as much. Most paint pigments have non-fading ratings. If you use a paint outdoors, the pigmented stuff is what you want. Unfortunately, many coatings makers use both dyes and pigments in the same recipe. The upshot of this is that your colors will change over time as the dyes give out to the sun.

Chris Marshall: I’m not sure what the advantage of non-pigmented paint would be on an outdoor wood project. Pigments help to block the UV light that eventually causes wood to turn gray and degrade. They also help to give paint its color longevity, as Tim points out. If what you’re after is a semi-transparent color that allows some of the wood grain to show through, I’d opt for an exterior-rated oil-based stain, because it won’t peel off over time and become a refinishing hassle. If you decide to go with any form of paint, make sure that your treated lumber is suitably dry (below 12 percent moisture content) before you paint it. The pressure-treated lumber I see at home centers is often so saturated with water and chemicals that it literally feels wet; if you paint over damp wood like that, you’re compromising the paint’s bond to the wood. More than likely, it will peel when the wood dries out. Personally, I’d give those slats six months to a year of drying time first before painting. After all, with treated lumber, there’s no harm in waiting.

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