I did this on a rectangular tarp and it worked quite well. The extra stitch made for a nice line.
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OK, now I am leaning towards just a hem. My fear was that it would be hard to keep the hem folded over.
I might try that tape.
Gah..I still can't decide. It's probably going to be here tomorrow, so I better make up my mind.
Well, two thoughts: 1) do a test hem on a small piece of fabric; there's nothing like hands-on experience, and 2) if you want an alternative to pinning, try spot-welding with a small soldering iron. The technique is to fold over 1/4" of fabric and hold it down with two fingers of your left hand (fingers spread about 3" apart) and lightly touch the fold with the tip of the soldering iron to the fold. It makes a tiny hole. As you hold the fabric down, spread your fingers apart so the fabric is slightly stretched - that keeps it flat. Keep the fabric flat for half a second while it cools and the two layers bond. Then move along the edge and fold the next 3 or 4 inches. I usually make the spot welds about an inch apart, doing two spots each time. (I almost never miss and burn my fingers. :)) Once the whole edge is turned and spot welded, it's fairly easy to fold it over again as you sew. This technique may sound tedious, but I can do it faster than pinning. YMMV. The work surface should be glass or formica, but doesn't need to be huge. Got an old mirror? If you really want to be careful, you can iron the 1/4" fold into the edge before spot welding, but eventually you'll skip that step. (Eventually you may skip all steps and just fold the fabric over twice as you sew - at least on some easy hems, but it pays to start slow and careful.) I have suggested this technique to others, but I'm not sure anybody's ever tried it, so don't feel bad if you don't. :)
My DIY hex tarp is just folded and sewn - no grosgrain ribbon edging. My Speer Winter Tarp is also just folded and sewn.
With silnylon, I like to use an iron to set the folds before I sew. The material doesn't stay folded, but the fold lines are there. I find that I don't need any pins because I have those ironed-in lines to use as a guide while I'm sewing. It helps keep the hems straight.
Jerry
Doesn't someone on the forum use a glue stick to hold the edges prior to sewing? I like the idea of "spot welding" the tarp. I don't know if my wife will let me set up her sewing machine in the garage though.
I tried the fabric tacking glue. It works but then is messed up my thread injector. The needle got all sticky and the bobbin case filled with sticky debris. :(
Not recommended (JMO)
On my DIY tarp I just used a rolled hem, on the edges, and I used small binder clips to hold the rolled hem before sewing. I also stiched the hem twice(evenly spaced apart) It seems to strenghten and stiffen the edgeges a bit. After all that I spray it with the silicone spray from WM (with the Blaze orage cap).
For those who have machines that can handle attachments / 2 screw holes to the right of the needle, I highly encourage spending some $ on a binder attachment. You can get cheap ones on ebay. They are cheaply made and are inexpensive. Sailrite and other marine upholstery places sell much better ones for more more money.
I have several of the cheap ones from ebay.
1" single folder handles 1" and 7/8" gross grain / twill tape / binding
3/4" single folder handles 3/4"
3/4" double folder takes a 2" or 2-1/4" strip and double folds it to a 3/4" tape. Great for making your own binding tape from scrap fabric or to match a project.
If you are making more than one tarp, you'll pay for the attachment with time saved. Pretty much feed the tape into the binder, hold the raw edge in the center of the fold and step on the gas. I can bind all sides of a tarp faster than most could even double fold and pin one side.
I also have a hemmer attachment, which is supposed to create a 1/2" double fold seam. In this case, either buying the cheap one bit me or I am doing something incorrectly, becuase I can not get it to work properly for more than about 1 foot before something goes haywire.