About 1/4 of the repairs we do are zippers.. I'm using a new thread, "ANEFIL POLY" made here in the US.. the size is 135 but because its braided, it looks like 92..
I can use a size 20 but find after awhile it starts to fray so I've gone up to a 21 or 22..
As for the zippers.. just a little hint... we charge between 8 and 9 dollars a foot to change out a zipper, so a 5 foot zipper would be about 45 dollars.. out of that 45 dollars, the zipper only costs about 3.50 for a 5 foot.. thats a #10 YKK...
Most of the cost is labor.. now a canvas shop has a lot of le-way when it comes to labor, and zippers are easy to change.. I can change out a 5 foot zipper in about 10 minutes but we charge the amount based on "ONE" zipper.. after about 3 or 4 setting on the table together, we can start to discount the cost..
A couple months ago, a guy came in and had his complete canvas gone over, (top stitching and zippers) If I were to charge him the normal rate, for 18 zippers and top-stitch, it would have cost him over $900.00..
I did the job in one afternoon, figured I was good to make $400.00 for the afternoon and he walked out paying the $400 plus the zippers at about $60 and he was happy as hell and I made good money
So dont feel bad about asking the guy for a discount for doing all of them at one time.. He can either get FAT on your job or make a decent wage.. ask him to cut the labor in half.....