Last month, we showed you how you could turn a big, expensive pile of lumber into a smaller pile of bulkhead sheathing and a large pile of sawdust. This month, we’ll show you how to transform that pile of sheathing into beautiful new bulkheads for your boat. Like making the sheathing stock in the first place, installing it is a tedious operation, full of opportunities to make mistakes.
Undoubtedly, some of your pieces of sheathing will have flaws in them-poor grain, poor color, or knots. As much as possible, you should work around these flaws. In new construction, lay out on the unsheathed bulkhead the locations of furniture, cabinets, hardware, or anything else that will cover up any part of the finished bulkhead. With a little planning, you can hide most of the material flaws in location that won’t show when the boat is finished, such as inside lockers, or settee cushions. Unfortunately, you don’t always have that option in a retrofit. That’s one reason for making extra sheathing stock: the inevitable, badly flawed pieces can go unused.
Surface Preparation
Prepare the plywood bulkhead surface for sheathing by power sanding with coarse (60 grit) sandpaper. If the bulkhead is teak plywood, follow this up with an alcohol wipe down to remove surface oils just before gluing down the sheathing.
Fastening
The tongue part of your sheathing stock has two purposes: to keep the joint between pieces of sheathing from oozing glue, and to assist in positioning and fastening the stock to the bulkhead. The bulkhead sheathing is attached using a combination of mechanical fasteners and adhesive. Epoxy resin, thickened slightly with a material similar in color to the sheathing (such as fine sawdust from making the stock, or a commercial thickener such as WEST System Tan Filleting Blend) provides the primary holding power.
Staples are used as mechanical fasteners and to hold the sheathing in position while the glue sets, but add little to the structural integrity of the installation. We recommend using an Arrow T-50 tacker with 3/8” monel staples. Do not use ordinary steel staples, and be sure to keep your monel staples clearly separated from any steel ones you have, since it is difficult to tell monel from steel without using a magnet.
You should not use screws to fasten the sheathing, except in areas where they are required to get the sheathing to lie down flat. Even carefully matched bungs in screw holes will detract slightly from the appearance of the bulkhead.
Fitting
The first piece of sheathing installed is the most critical. It must be absolutely plumb, since the sheathing consists of a series of vertical lines which will look ridiculous if they don’t line up with the other verticles on the boat, such as door edges.
Begin next to a doorway to another true vertical, if possible. This allows you to clamp the first piece into position before gluing, so you can carefully line it up. It is best to overlap the doorway slightly with the first piece, since a joint in the sheathing which lines up with the edge of the doorway will look odd. You can trim off the overlap with a saber saw after the doorway is sheathed.
The tongue edge of each piece of stock is the working edge-the edge against which the next piece will be fitted. If you try to do it the other way around, you won’t be able to staple each piece into position.
Cabin overheads are almost always cambered in section to avoid a boxy look.
This slightly complicates fitting the sheathing tightly overhead. An adjustable bevel gauge can be used to take off the angle between the vertical simple thin cardboard template the width of a piece of sheathing, trial fitting to the top of the cabin using a mat knife and a straight edge. Cardboard templates are also useful when it’s necessary to fit around permanent joiner-work. It’s better to make cutting mistakes on cardboard than expensive wood.
Fitting at the bottom of each piece is the same as fitting at the top. Once again, patterns are helpful. If you’re sheathing a new bulkhead, you may have to relieve the back of each piece of sheathing to fit over the tabbing which holds the bulkhead to the hull. This can be done with a plane, chisel, or belt sander. In some cases the tabbing may be so thick that you can’t cut away enough material on the back to make the sheathing lie perfectly flat. You can get away with having it sprung out slightly, since the edges of bulkheads are usually covered with trim, or are hidden inside other furniture.
The top and bottom bevels are cut with a band saw, saber saw, or coping saw.
When you’re finished with the position of the first piece of sheathing, clamp it firmly in place on the bulkhead along the working edge of the sheathing as an alignment reference. For the second piece in the same fashion as the first, once again using clamps to hold it in position after fitting. From here on out, you won’t be able to directly clamp pieces in position. Instead, clamp a batten to the bulkhead long enough to reach over the piece of sheathing currently being fitted. A thin wedge under the batten will apply enough pressure to the sheathing to keep it from sliding out of place as you work.
We have found it best to install no more than four pieces of sheathing – about a foot of bulkhead width – in any one gluing. More than that becomes awkward to glue, clamp, and staple in place. It’s better to do the bulkhead right, a little at a time, than try to save time by doing it all at once.
You can fine tune the fit of sheathing at the overhead and bottom using a sanding block wrapped with coarse sand paper as a rasp. This is less likely to cause splinters than trying to use a tool such as a block plane or a Surform, and allows subtle shaping of the top to conform with the curve of the cabinet top.
After the last piece of sheathing for the first gluing is fitted, strike another pencil line on the bulkhead along the working edge of this piece. This will indicate the edge of the section of bulkhead to be painted with adhesive for the first gluing. Number the pieces before removing them from the bulkhead, either writing the numbers on masking tape stuck to the surface or by lightly penciling numbers on each face.
Gluing
You’re now ready for the big test.
Since mixed epoxy resin has a short pot life, get everything ready before mixing up the glue. Lay the pieces of sheathing on a table covered with polyethylene sheeting, to which dripped resin will not stick.
Use either a foam brush or foam roller to coat both the back of each piece of sheathing and the face of the bulkhead. Coat both with unfilled resin first, then mix thickener into the resin and brush another coat onto the back of each piece of sheathing. The thickened resin should be the consistency of honey, so it can fill any gaps between the sheathing and the bulkhead surface. It should be brushed out fairly evenly, leveling with a wide putty knife if necessary.
Take your time to position the first piece of sheathing, being careful to line it up with the line you drew on the bulkhead as a reference mark. Use a couple of C-clamps to hold the piece of sheathing in position. Be careful, since the low friction resin-coated surfaces will want to slide around as you tighten the clamps. Use thin pieces of clean scrap wood under the clamps to keep from damaging the surface of the sheathing.
When the first piece of sheathing is clamped into position, staple the tongue to the bulkhead, using firm pressure on the tacker to driver the staples in as far as possible. The staples should be placed about a foot apart, as far in from the edge of the tongue as possible, with the long axis of the staple parallel to the edge of the tongue.
Even using a lot of pressure on the tacker, the staples will probably not drive all the way into the bulkhead. Use a ¼” diameter pin punch and hammer to set them flush with the surface of the tongue.
Aligning the next piece is easier, since it is simply butted up against the edge of the first, then stapled down. After stapling the edge of the second piece, you’ll notice that its groove edge – which is not stapled down – has probably lifted slightly, and is not lying down flush. This edge, and the corresponding edge in the next pieces, will be wedged down after all the pieces in a single gluing are stapled in place on the bulkhead. Now you see why it’s best to fit only a few pieces of sheathing at a time.
Attach the next piece or pieces in the same manner as the second.
You now must face the problem of making the sheathing lie down flat. This is accomplished using a number of battens about ¾” square, which can be made from any type of clean scrap. These are clamped to the edge of the bulkhead at the doorway, allowing them to overlap the last piece of sheathing on the bulkhead by a couple of inches. Drive a screw through the batten into the bulkhead just past the last piece of sheathing. The screw hole in the bulkhead will be covered by the first sheathing strake of the next gluing.
While the batten will do a pretty good job of flattening out the sheathing, you will probably have to use thin wedges or shims under the batten to really hold things down flat. By pressing on the face of the sheathing with your fingers, you be able to feel any sections that are not lying down flat on the bulkhead below. Add more battens and wedges as necessary to hold things down firmly.
Now clean up the mess of glue that has probably squeezed out between some pieces of sheathing and at the top and at the bottom of each piece. A rag dipped in stove alcohol or shellac thinner will remove gobs of epoxy from your sheathing. Use a putty knife to clean up glue that has squeezed past the edge of the last piece of sheathing, to leave the bulkhead clean for fitting and gluing the next the next pieces.
Cleanup is critical. The more effort you put into removing glue while it is still wet, the less work you will have when finishing the surface after the bulkhead is completed.
Always wears latex exam gloves when working with epoxies. A box of 100 costs about $1.5, and it’s the best $15 you’ll ever spend. Epoxy sensitization is permanent, and miserable. Wash any epoxy which gets on your skin off with soap and hot water before it sets up. Don’t use solvents to clean your skin unless absolutely necessary, and then use alcohol, not anything more powerful. Even alcohol has a serious drying effect on your skin.
Carrying On
Successive fittings and gluings generally get easier, although your hold-down battens must get longer and longer as you go.
A few screws through the sheathing itself will be inevitable, as it will undoubtedly be impossible to wedge some pieces completely flat, particularly at the bottom edges and at the extreme edges and at the extreme outboard edge of the bulkhead. Use as few fastenings as possible, and make them fairly large-diameter panhead stainless steel self tappers driven only flush with the surface. After the glue kicks, the fastenings can be removed and the screw holes counterbored and bunged with plugs cut from the same material as the sheathing. Do not use readymade plugs of what is nominally the same material as your sheathing, since the readymade plugs may b different in color from your own stock.
Work from roughly the centerline of the bulkhead all the way to one side, then work from the center to the other side. You will have to reverse the direction the tongue of the sheathing points when you begin working from the center toward the opposite side, since the tongue would otherwise be hidden. Do this by first gluing a thin “tongue” under the grooved edge of the first piece of sheathing you installed. You’re now ready to start gluing in the other direction.
Finishing
When you’ve finished installing all the sheathing, your bulkhead will look pretty sorry. It will probably have edges which don’t lie flat, gobs of glue here and there, and torn out surface grain. The effort you will put into finishing turns this rough bulkhead into a thing of beauty.
Begin by machine sanding with coarse paper, gradually working from 50-60 grit down to about 100 grit. Use an orbital sander such as the Makita B04510 or the Porter Cable 330 Block Sander. You can use a block plane to cut down any badly raised edges which weren’t completely down. Don’t worry about them coming loose. Some voids behind the sheathing are inevitable, but they won’t affect anything.
After machine sanding, you must finish sand by hand, using a flexible rubber sanding block of the type used in auto body work, or a homemade block of cork or neoprene. Go back to 80 grit paper, sanding with the grain. Sand out any machine sander swirls on the sander swirls on the surface with 80 grit paper before moving on to 100, then 120 grit, but highly-figured wood must be further sanded with 150 and 180 grit to remove fine scratches.
Varnishing
We prefer a satin finish on interior woodwork. It hides flaws better than gloss varnish, an looks more like a traditional hand rubbed finish. If you’re a real masochist, varnish with gloss varnish until the grain of the wood is completely filled. Then rub down the surface with powdered pumice on a cloth dampened water or furniture oil. You’ll eventually get a beautiful surface unmatched in appearance, but at the cost of untold hours of labor. We’d suggest you try a hand-rubbed finish on something small, like a magazine rack, before committing yourself to doing something as large as a bulkhead this way.
For most of us, a satin interior varnish is more than reasonable substitute. Use at least five costs to fill the grain. Sand between each coat with 180 grit paper, using a tack rag to clean the surface just before applying the next coat of varnish.
Odds and Ends
In new construction, the edges of bulkheads are covered with a margin piece to hide any fitting errors or rough edges. The margin pieces should be of either the same wood as the bulkhead, or of a contrasting wood. The margin pieces should be fairly thin, and must be carefully fitted to the side of the hull. If ceiling is to be installed, margin pieces may be unnecessary, as the cleats for the ceiling will probably hide rough edges.
In retrofits, either reuse the original margin pieces or make new ones of the same material as your new bulkhead, using the old pieces as patterns. The margin pieces should merely be screwed on, either counterboring and bunging flathead screws, or countersinking oval head screws.
Sheathing bulkheads is not a beginner’s project. But with careful planning and cautious execution, it can give your boat an interior unmatched by the stick, run of the mill plywood bulkhead. And it will give you a great deal of pride in your boat.
