A Hotknife for the Rest of Us

When I built my first canvas project, a mainsail cover, I cut out the pieces using scissors as usual. Later, when I made the cabin cushion covers, I discovered how much better a hotknife works for cutting synthetic marine canvas like Sunbrella. Instead of the edges unravelling, they fuse solid. That not only makes them easier to work with and prevents getting pieces of thread all over the house, but it also ensures that they won’t come loose after years of use and abuse.

Sunbrella marine canvas cut with a soldering gun (left) and scissors (right)

Rather than buy an expensive industrial hotknife, I made one easily from a cheap Harbor Freight soldering gun. I heated one of the tips until it turned red and then gently bent it into the shape of a hot knife blade and lightly hammered it flat. The tips are made of chrome plated copper, so they bend easily. The makeshift blade heats up fast and cuts quickly.

Unlike scissors, you can use a straightedge with a hotknife to make perfectly straight cuts. I have a leftover pane of glass that makes a perfect burnproof cutting surface. Cutting this way produces a little smoke from the melted Acrylic that can be irritating, so do it in a well ventilated area.

A hotknife is also handy for making clean cuts in any synthetic material like zippers, hook and loop fastener tape, sail cloth, webbing, and yacht braid.

Here’s a tip for cutting rope with a hotknife. First wrap the rope tightly with tape around the point where you want to cut it and then cut through both the tape and the rope. After the cut cools, remove what’s left of the tape. The tape will hold the rope strands together while the hotknife fuses the ends. This makes a smaller cut end that is easier to pass through blocks, deck organizers, fairleads, and so on.

Now, Harbor Freight tools have never been known for their high quality and their soldering guns are no exception. I found that the tips overheat easily when used continuously such as for cutting canvas and the case can begin to melt around the electrodes (see the picture above). For that reason, if you plan to buy one of these and cut a lot of canvas, this is one time that it might be worth buying the extended warranty. I minimized this problem by learning judicious use of the trigger. Only keep the gun as hot as you need to for as long as you need to make a cut, then let it cool a little while you prepare to make the next cut.

Also, the plating on the tips oxidizes and they quickly lose heat conductivity to the point that they won’t cut anymore. I solved this problem by applying a little heat sink grease that I already had on hand around the ends of the tips where they slide into the electrodes. It helps a lot. You can get heat sink grease at your local Radio Shack store.

Since making this hotknife, I’ve used it for numerous projects including: running rigging linescabin cushion covers, mainsail cover, foredeck sail bag, bimini covercrib board storage bag, and lifeline cushions.

The Bottom Line

Suggested price: $139.95
$tingy Sailor cost: $14.95
Savings: $125.00


If you like this project, then you’ll really like my ebook Do-It-Yourself Small Sailboat Canvaswork. It contains this tip and nine full canvaswork projects with dimensioned drawings, all for only $20 USD. Click the picture at right to order your copy for immediate download and get started today!



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9 Comments Add yours

  1. captnmike says:

    Seattle Fabrics have a hot knife that is based on a soldering iron – cost under $ 40.- http://www.seattlefabrics.com/scissors.html – not as low cost as yours, but a bit less work – if you have the proper base soldering iron, you might be able to buy a replacement tip – I think Sailrite has a tip with a 1/4 inch shank that will fit into some soldering irons

    Not as low cost as your solution – the hot knife from Seattle Fabric is I think the same one I bought years ago and it worked fine – eventually I got the fancy Englow (sp?) with a foot for the knife so I did not need to worry about burning up the surface I was cutting on

    1. Thanks for the tip, Mike! A pen style hotknife would indeed be more convenient for some folks or projects than a gun style one. And I had forgot about Seattle Fabrics. I came across them while I was shopping for some sail cloth awhile ago. It’s good to have alternatives to Sailrite for marine fabrics.

      1. captnmike says:

        Perfect Fit (http://www.perfectfit.com/) – in Tuckwilla / Southcenter is another good place – good prices – the site says wholesale but they have sold to me when I just walked in. Do some homework before going down as they don’t have a “store” area as such – they are a large warehouse operation – bit of a drive for me – but you on Mercer Island are closer – I always save up and get as much stuff as I can on one trip to make the trip worthwhile – the folks were helpful when I went in

      2. I’ll make a note of them for the next time I’m over there. I bought Summer Dance on Mercer Island but I live in eastern Washington and make an occasional trip over to the wet side.

      3. captnmike says:

        What part of Eastern Washington? I used to live in Walla Walla and still have relatives in the area & in Pullman

      4. About 10 miles NW of Spokane near Colbert. I did some work in Walla Walla years ago. It looked like a nice community. Been thinking of taking Mrs, $tingy down there for a wine tasting tour. I’ve heard it’s good.

  2. Tomas Kruska says:

    Hehe… nice idea. I had the completely same a few months ago when I was “cutting” the canvas for the window shades.

    BTW. I’m glad you fixed the the boat after the storm and saved her from scrap 🙂

    1. Hi, Tomas

      I just visited your blog and left a comment. It looks like your family really enjoys vacationing on Sole Mio. Have fun working on her!

  3. Desirer says:

    What a cool idea! Heading to home depot now…!

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