Re-stretching canvas after filler applied?

Steven Hanton

Curious about Wooden Canoes

Apologies if this has been covered, but I did search thoroughly and don't see a specific discussion of this....

My issue is that I canvassed and filled a hull when the temperatures were low and the humidity high. Now as the temperatures have warmed up, the hull has shrunk dramatically away from the canvas, leaving the canvas very baggy along the gunwales and in parts around the stem. I'm sure the hull will pick up some moisture again and swell out a bit, but I am wondering if I can re-stretch and tack the canvas again even after the filler has now cured?

One other thing.... the canvas just under the gunwales, between where it is fastened to the rib tops, has developed little scallops - it is baggy for about 5 inches below the gunwales. I read in 'Building the Maine Guide Canoe' that over stretching here can cause this after the canvas relaxes after filling, but I have also seen on the forum here that Mr Thurlow states that in 30 years he has seen no known issues related to over-stretching. Can any one clear this up for me please?

I've experienced the high humidity thing, too on a couple of occasions. Doors, cupboards, etc swell with humidity...why not wooden canoes!
Once you think the humidity is lower and the hull shrinkage is done, pull the staples/tacks and re stretch.
Both times it happened to me, I had already trimmed the canvas to the plank height. If you have done this, you can grab the canvas with a pair of pliers and re stretch. Trimming the canvas is now the LAST thing I do after all repairs, paint, filler, and varnish.
It was amazing to me,...first that the humidity affected it that much, but also how little stretching it took to tighten it back up....
Best of luck
Me too

I thought I was the only klutz who made a mistake stretching canvas. My STL arrowhead sat in the shop all winter and into spring. The humidity is high and the temperatures as well. This is Tennessee. When I stretched the canvas, outside between two trees, I noticed the longer I worked on it the more the canvas relaxed or stretched. I kept tightening the winch. I filled the next day and two days later took the canoe in the house to hang from the living room ceiling and cure. Now, Two weeks later I have the same problem as you describe.
Can a canvas be stretched too tight? Yes it can. I tore the last one due to too much tension. I noted that problem last year.
Tell me how the re-stretch goes. I might do the same, or just leave it.
Thanks for your responses guys.

I live in Scotland and my workshop is in an old, damp, stone barn fairly high up on the side of a hill. It has no heating and no windows! Our climate is not great, with alot of rainfall and so damp and humidity are my greatest problems. Indeed, when I canvassed/filled this particular hull, the temps in my workshop would have been around the 4 deg C (sorry, no deg F here!) mark for the previous 5 months or so, with high rainfall and consequently high humidity. We have had an impressive (and unusual) spell of +20 deg C and sunny weather for the last 2 weeks or so and hey presto, the canvas has relaxed considerably. I had considered the humidity/shrinkage problem before, but this time has been the worst so far. As you mentioned Dave, doors etc can swell/contract alot and so it makes sense to me that a wooden hull with such a large surface area will do too. I think I'll be careful to stretch canvas when the humidity is lower then it will likely only expand when humidity decreases. Of course I'm sure my stretching technique could be improved, but 7 boats in, and with almost no builders on this side of the pond, I'm simply learning from my mistakes as I encounter them. Best way I suppose, but can be frustrating.

I'm away for the next couple of weeks, but I will take before and after shots when I attempt a re-stretch on my return. I have also just canvassed and filled another boat during the hot/dry weather of late, so will be interesting to see the comparison if any.
One possible way to ensure tight canvas is to soak it with zinc naphthanate (anti mildew treatment) before filling.
But you might not be able to get it, as there have been government imposed restrictions for health and environmental reasons. I know it is difficult if impossible to buy in the U.S. (federally regulated?), but we can get it at any hardware store here in Canada.
Pardon me if this is covered somewhere I can't fiibd an answer. I was reading above about humidity effecting the overall canvassing out come. I'm a Newbie with only 1 other restoration under my belt 40 years ago!anyway I've asked my question already but I'm getting anxious I'm a bit ocd. I stretched and filled my hull it sat for 4 weeks with out a hitch no bubbles ripples nothing all looked great until I got down filling the sponsons 2 days ago
Oops, a day after the sponsons were filled air pockets formed between the end of the sponsons and the stem today its worse. We did just have 2 powerful nor easters in a row 1 ending early this morning The odd thing is I had the garage door open today with the sun shining on the stern today and they almost disappeared as soon as the sun and temperature went down this evening they came back even worse than yesterday . Can someone tell me something. Should i wait it out or attempt to restrech the ends while the filler is not fully cured? I don't know if I should burn it or try to save it somehow. I'm not sure my hands can take recanvasing, help
Be patient. I don’t see anything that can be done right now. Allow the sponson filler to cure. It is possible that when the solvents evaporate from the filler, the canvas may shrink. If it still has the pockets when cured, pull the tacks or staples and try to pull it tight.
This is the worst side. They disappeared when the warm sun shined today at 50 just took this it's now 34. I'm enclosing a before and after image. Somehow ones upside down but im showing the same end.
Dave, I’m somewhat of a new member and I’m preparing to recanvas a ‘38 OT. The boat has been in storage for 30 years and dry. I’ll use linseed turp mix before canvas to add some new life to the cedar planking but this thread has me considering a few questions. I’m in Michigan and spring is coming with increasing humidity. I’m working inside in a heated shop with low humidity now. I wonder how tight should the canvas be stretched to avoid this problem? One other concern is that I purchase the canvas in 1980 from the old Town company. Don’t laugh I had the best of intentions but life got in the way. There are probably going to be fold marks . I’m thinking of leaving it stretched for a while before putting the boat in the envelope and once inserted let it sit awhile before finally tacking. Your thoughts? Should I purchase new canvas or go for it? Also how much filler do you think I’ll need? It’s an 18’ otca. I’ve purchased one gallon from Kirby paint and I’m thinking I’ll need part of another gallon to get a good result.
Any input would be greatly appreciated.
Tony, there is a related link that might be worth following...the discussion about this boat has been split into multiple threads...

I'm right on the fence with Dave about what to do next..clearly the filler is/was not ready to be worked. Possibly the canvas was not tight enough and the application of more filler when the sponsons were added highlighted this.
My advice to the chief had been to sit tight and be more patient..but I also suggested that he tighten it where he can. After more thought and seeing Dave's second post I'm trying to decide the risk/reward from reopening the canvas and attempting to re-tack it... That's such a high risk move to make given that there is no longer enough canvas to pull, what is there is loaded with bedding compound and filler and wont want to lay flat etc... it all has the making for a real mess...but on the other hand, if it stays the way it is, what is there to lose??? That canvas is not going to magically sort itself out....
Not sure what to say about canvas that is 38 years old. If it has been stored well it may be ok to use.
The folds should come out when stretched. If not, try using a steam iron while it is stretched and before you pull it tight and attach it. Letting it sit while stretched may help as well.
I use the “upside down” method when stretching. It is amazing how much the canvas stretches. In my experience, it stretches about 1 inch per foot of canoe. So in your case, the canvas would stretch 18”s!

How tight? I’ll just say tight as you can in a humidity below 50%. In summer, I use a dehumidifier in the shop if I’m planning to canvas, or just wait until the humidity goes down.

You can buy treated canvas if you decide to buy new. In days past, there were some applications that were available to treat canvas, but they are now banned in the U.S. Most of us add or use mildewcides to the filler.

For traditional filler on a 18’er, I’d opt for a gallon and a half. There is nothing like running out.
I’d rather have extra.
There is a lot of information on this forum about these issues. The search function is your friend....
I can remember two occasions in 19 years of restoration where I had to restretch canvas. Both involved canvassing in high humidity. The first was on a humid foggy morning on the north shore of Lake Superior. The other was under a tarp in a rain storm for a demonstration at a WCHA event.
The first one was filled on the same day..
A week later I noticed the sag and bubbles. I did unbutton the ends. There wasn’t much to grab on to, but enough to restretch it tight. I recall using flat nosed electricians pliers. I think I pulled an extra half inch out of it. Not much, but enough to pull it tight. As Mike says, there is a risk. But what have you got to lose?
If the end isn’t so pretty when you are done, apply some fairing compound and smooth all out..

If it were mine..... I would wait until low humidity when the canoe is at its driest, then restretch.
Thought I'd report back after much thought even more patience and the help found here. I'm pleased to say alls dry and it's tight as a drum. The stress was high after so much work. This is what it did. The air pockets were worst on the ends both were on the side of the 2nd flap that's pulled over the first. I daringly and carefully pulled tackes and was able to tighten up somewhat. I only had to remove about 6 tacks at each end. I then used a tip Rollin gave me that really saved the day! A hand held steamner. I attached a drinking staw to the steamer with good old duct tape. The straw fattened and I carefully slid it in between the canvas and the planking in the troubled areas. Also had the canoe upside down so the stem would rise. Batabing! Now it's filled looking great and can sit and cure until April or May and my project should be completed on schedule. I want to do the paint and varnishing as soon as spring breaks. Before it's to hot, that's the one thing I'm experienced in. Heat make finish coats dry way to fast. I always Get best results out of the sun when it's around 60-65. I guess if you spray the temp may not be as important. Thanks all
I'll sure .Thanks all
It looks like the USS Nautilus without it's tower


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