120 is a large needle and 45 is thin thread.  I used 100 and 110 needles with 69 thread.  I did have to change to bobbin more often.   You can use this as a guide http://www.tolindsewmach.com/thread-chart.html and play with the needle size. What kind of machine are you sewing with.  Other ideas...

• Paul's thoughts on the size of the hole in the needle plate and adjusting the tread tension super low are right on.  You are doing zig zag, so you have a slot in your plate, but the same applies.  You may be able to use silver solder to narrow the slot if necessary.
• Use a large foot on your machine.  I tried a roller foot without luck.
• Check timing of the needle vs the feed dogs.  Check feed dog height.  Make sure your machine is to specs.  
• Consider rolling the seams with a pipe or tube after taping to better help hold the layers together.  
  
• When taping, stretch the lower and upper cloth equally such that the tension matches - in your pictures you are already developing some wrinkles when the cloth is just tapped.  If you are using pins to hold the lower cloth down, pre-stretch it and then carefully apply the upper seam layer.  Release the pins and see what you got.  Repeat... 
• Large stitch length helps - go to the max your machine can do.
• 10 mm wide remnants, about 500mm long are sufficient to practice on and tune your machine.
• The cloth is hard to sew!  I struggled.  My sail maker struggled with his project.  Very slight cloth pucker will come out under tension from the wind.  You got quite a bit though and I definitely understand your concern.
• You have a triple stitch zig-zag.  Can you get away with a single row rather than two.  That would decrease your puckering. 
• WeatherMax is very elastic cloth - try not to sew it on the bias https://www.canvas-boat-cover-and-repair-advisor.com/how-do-you-stop-the-puckering-on-weathermax-seams-comments.html
• Guide by the manufacturer: https://issuu.com/challengesailcloth/docs/challenge_sailcloth_weathermax_prod

Let us know how things turn out.

Hard Perk wrote:

Arne Kverneland wrote:

Hard,

how thick thread  and needle are you using?

Are you able to reduce the thread tension, both on the under-thread and over-thread?

It looks to me tha slacker thread tension is needed.

Arne

The needle was 120 and the thread 45. They use similar materials often, also repairing sails. The machine is a professional one in good shape. We'll try out some different approaches tomorrow. 

Possibly  to much tension. The other thing to look at is the size of the opening that the needle goes through on the needle plate. If you are using a professional sailmaking machine then that hole is probably rather large. Not a problem with stiff sail cloth but Weathermax is soft and dimples with every stitch...

A smaller needle and or closing up the opening a bit should help. Plus the minimum amount of tension that will work. The seams are the most difficult, once you have three or more layers, things go better.

Hard,

how thick thread  and needle are you using?

Are you able to reduce the thread tension, both on the under-thread and over-thread?

It looks to me that slacker thread tension is needed.

Arne

The blue end of the spectrum reflects UV, and remains more colourfast; the red end of the spectrum absorbs UV (so the dye fades quicker), but this doesn't necessarily mean that the cloth breaks down. White (the absence of dye) both lets all colours of light, including UV, into the heart of the fabric and reflects all colours of light equally. I wouldn't worry too much about cloth breakdown in high latitudes, but I would certainly avoid white in the tropics, for both long life and crew comfort.

Asmat Downey wrote:

Grey sea, grey sky and murky visibility make a white sails disappear while a dark colour continues to stand out.

Not to mention the yellow!