Haven't used the Serafil thread myself so can't comment on its strength or UV resistance. For my 4oz & 5oz Dacron sails I use the V69 Dabond, for 6-7oz sailcloth I'd go with a V92 thread and anything heavier then that I'd use V138 thread. See
www.sewingmachinesaustralia.com.au/shop/buy-accessories/sewing-threads/coats-dabond-uv-treated for more info.

have ordered some dabond v138 black and white,will see how it goes and report back on ease of use, Give me a few years and can report on longevity as well!

V69 would have been ample. V138 is going to add a lot of weight and slow you down!

I probably should have added a smiley face! V69 is more than ample for a 30 footer. V138 is going to stand proud of the cloth a bit. Just a quick look in my sewing room and the heaviest thread I have is V92.

www.sewingmachinesaustralia.com.au/shop/buy-accessories/sewing-threads/coats-dabond-uv-treated
They don't list V92 on the website, but I've bought it from them several times.

I find that V69 only lasts a couple of years or less of continuous exposure in Qld. Maybe not so important for work on sails, but for covers I always use V92 or V138 to maximise UV performance.

Yes, Tenara does seem to be the ultimate, although I haven't found a reasonably-prices source in AU. Apparently you do need to go to a fair bit of trouble to set the machine up to use it.

Our local canvas shop has told me they charge an extra few hundred dollars for anyone who asks for Tenara, just to cover the setup PITA. On the other hand, from the shop's point of view, it's in their best interests to discourage customers from using Tenara because they'll never get a re-stitch job. I think the re-stitches are a pretty reliable source of recurring income.

Ramona said..
I have found using straight stitch instead of zig zag on my sail cover it has lasted much better. I have been using a boat trailer winch strap for years I stitched with v69.

On winch straps you should try v138 , has over 10kg breaking strain and will outlast the v69 a few times over. Or else try some v207 in the Speedy Stitcher, should last the life of the straps.
I used v138 on my front tramps on the cat and despite being full time in the sun , salt water spray and walked on for the past 2 years they still look as good as the day I fitted them.

andycass said..
just got the cones of 18 wt/v138 thread and ran some scrap canvas through the machine went well once i put a fence post in for a needle. I reckon the extra weight and wind drag will slow me down, at least i can use the surplus thread for spare rigging! It looks like rope compared to my usual 60wt or 40 wt thread. Definitely will outlast my sails, probably outlast me too.

Apart from the thread showing, the holes will have quite an effect with the sun shining through when you gaze up to admire your handy work.

I have an old Singer treadle sewing machine with the same needle as my industrial Singer machine. With the belt twitched up tight this will drive through about 4mm of cloth with ease. The industrial machine balks at it. Only straight stitch though. Impressive bit of engineering from the early fifties.