Thursday, December 11, 2008

Stretching Canvas

One of the things that I've found in talking about cedar and canvas canoes with people is their absolute terror at the thought of replacing canvas on a canoe. I don't know why that is, but it seems to be one of those psychological roadblocks that keeps people from restoring and repairing their canoes. It's not that difficult. The big downside is waiting for the filler to cure once the actual canvas work is done.

Before you start, you need to take off the old canvas. This usually involves removing the outwales and removing the tacks hidden beneath that held the canvas in place. You may also have a keel and brass stem-bands to remove. Typically, all of the fasteners are found along the gunwales and from stem to stern at the centerline of the canoe.

Once the canvas is removed, you will want to inspect the woodwork to see if there is rot, cracked planking or ribs, loose tacks or any varnish work that it needs. If there is such damage, now is the time to repair it.

Once any necessary repairs have been made, make sure the outside is smooth and fair and give it a light sanding and a light coat of linseed oil and turpentine. This helps to waterproof and preserve the wood. When done, the boat looks like this:

You should also have your materials collected. You'll need a few specialty items and a few tools.
  1. Canvas - It should be an appropriate weight for the boat your building - typically #8 -heavy, #10 - Typical or #12 - lightweight and longer than the boat by about a yard or two.
  2. Brass Tacks - AKA Canoe tacks - in two lengths - 7/8" to 11/16" for the at the ribs and 5/16" or 3/8" for the stems.
  3. Bedding Compound - a peanut butter consistancy material for the stem ends.
  4. Filler - a thick paint mixed with or without white lead for "killing the weave" on the canvas. More on this later.
  5. Brass Stem Band and Screws
  6. Cardboard
  7. Prop sticks or weights (sandbags, garden fertilizer, etc.)
  8. Winch - AKA "come-along"
  9. Chain and hooks
  10. C-Clamps
  11. ~3' Blocks with "key" for clamping canvas as stretched and eye bolts to pull canvas. - one set for each end of the canvas.
  12. Canvas "clothespin" - a piece of hardwood with a wide kerf cut up the middle (lengthwise) and a bolt to prevent it from splitting at the closed end of the kerf.
  13. Canvas Stretching Pliers
  14. Small Hammer
  15. Razor Knife
  16. Screwdriver
First, you will need a place with two solid attachment points. These can be trees, sturdy posts or the like. They should be far enough apart to allow you to put the canoe between them with room to spare. They should be strong enough to take the horizontal force you will apply to them. In the picture below, you can see one of the attachment points in the wall to the left of Neil with the come-along hanging from it. He has also hung the keyed blocks from the ceiling by purple straps. It is a good idea at this point to have a clean floor to work on, so the canvas stays clean and more importantly, you don't get debris between the hull and the canvas because when it is stretched anything between the two will show.

After cutting the appropriate length of canvas, fold it in half the log way with the opening at the top and clamp both ends with the keyed wooden blocks. The key keeps the canvas from slipping out under tension. The blocks are clamped together using two or three C-clamps. When this is done, pull up the tension on the canvas to make sure there aren't any wrinkles.

Let the tension off and put the boat in the envelope of canvas. Think of it like a hammock for your boat. Be sure it is well seated in the canvas. Pull up on the tension again, but not so much that you rip the canvas or it slips from the clamps. You will then need to either weight the canoe with bags of something heavy or if you have a ceiling overhead, you can use some wood to prop the boat down from the ceiling. The best way is to protect the inside of the boat with cardboard, put a 2x6 in the bottom of the boat and use another piece of lumber to wedge between the 2x6 and the ceiling joist. Install the canvas "clothespins" at either end as close to the stems to hold the canvas near the hull at the ends.

The longer tacks are then put in. There are two at each rib, just below the top edge of the planking. Usually a scrap of wood is used to protect the inwale. You then grab the canvas with the canvas-stretching pliers and rock the handles into the hull on the block of wood. (In the picture below, the pliers are upside down - see the rounded top?) Placing tacks is done by Braille - you need to feel for the rib location and the top of the planking - sometimes you can see the rib through the canvas. You'll want to be about 1/4" down and 1/4" in from the edges so you don't split planking and ribs. If you get the tension correct, there will be little puckers above the tacks. Make sure any wrinkles in the canvas are being moved from the center of the hull towards the stems. Tacking the canvas is most easily accomplished with two people.

Alternate sides to keep the tension on the canvas even. If there are wrinkles, pull up on the canvas a bit harder. As you get close to the ends, you will also find that you need to trim the canvas near the gunwale so that the pliers can "bite" the canvas for a good grip as there will be too much fabric. You will also find you have to pull the canvas towards the stem to get rid of the wrinkles. When you get all of the tacks put in at the gunwale line, you need to make a choice. You can:

A) Simply let down the tension and cut the canvas (vertically) about a foot beyond the end of the stem. The excess canvas helps you to pull the fabric around the stem to finish this area.


B) Roll a scrap of canvas to be a small "sausage" about 5 layers thick an inch wide and about 4' long (depends on stem length) and staple parallel to the stem through the "sausage" and the canvas into the planking and stem on both sides of both stems from deck to keel. Then cut down the hull as above. You will need to work any wrinkles out as you staple this in place.

The advantage of B is that as you're tacking the stems in place, there is not tension. This is an advantage if you are working alone.

Put the canoe upside down on a pair of saw horses and remove the clamps.

Before you tack the stem slit the canvas down the middle of the envelope. You start this cut at the keel where the fabric would overlap. Put one tack in the stem just above the slit. Then, apply a thin and even layer of bedding compound over the stem and pull the canvas off "normal" to the stem so you get no wrinkles. Starting at the keel and heading towards the deck tack every 3/4" or so. It should look like the the picture below. The wrinkle to the right of the stem shouldn't be there...

Trim the excess canvas close to the stem on the that hasn't been tacked. Apply bedding compund again and repeat in the other direction. Trim the excess canvas again and repeat for the other end of the boat:

The next step is to apply the filler to the canvas. The filler is a tough, abrasion-resistant coating that fills the weave of the cloth and effectively waterproofs the boat. There are leaded and unleaded fillers. I personally prefer the leaded filler for the anti-microbial properties of the white lead. If you avoid contact with the lead, you should do OK. I think it's nice to label the inside of the boat somehow to let a future restorer know that they are dealing with lead... The leaded and unleade filler apply almost the same way, but the leaded tends to inhibit mildew in the canvas for many, many years.

It's applied by brushing on with a heavy brush and then rubbing in with a canvas mitt. (Wear latex or nitrile gloves underneath if it's leaded filler!) It may take several coats. You'll know when to rub with the glove when it rubs smoothly and leaves you with a slate-like finish.

Whe the filler is applied, it will take a few months (yes months - 2-3!) for the filler to cure or "polymerize". When cured, you can sand (wear a mask!) and apply primer and paint - usually a porch and floor enamel and re-install keel, stem band and outwales.

Happy Paddling!

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