Size and (more importantly) the amount of shape in it has a big effect on how thin you can go although the base and/or 'tunnel' would typically 16swg even if there's enough shape in the thing to go thinner without losing stiffness/dent resistance. If MIG is your only option then i wouldn't go thinner than 16swg anywhere regardless of shape as getting airtight welds on 18 or 20swg will be a lot harder
I made one from 1.2, but put internal bracing & baffles in as it was quite wide. Thicker is definitely easier and less tedious though at least with MIG, if the panels are flattish. Seen them made with rod 'frames' and infill panels too. What sort of shape are you going for?
Thanks the advice. I was planning on having internal baffles, to help with deformation. The tank will be pretty single in form with straight faces. Looking forward to getting started on it.
I'm having the sections laser cut, so was thinking of adding tabs and notches for alignment and location. Any other tips to help?
Sorry, forgot about this. Little angle templates and suchlike to check each side as you go along, and maybe the baffle would do some of that job. That's what I did, plus squares off the table, anything that lets you check things.
Tack panels before finish welding, so you can tweak things.
Where I could on mine I welded inside as well, before adding the top panels and after doing the outside; could run beads instead of spotting that way, and stops leak chasing on at least some it..
There's a recent thread on here on sealing tanks too (while you got the raw panels you could degrease and sand the insides, would help resin to get a hold).
You shouldn't need baffles in a bike tank, it should slope and take fuel from the lowest point. 18g metal, about 1.2mm thick should be plenty thick enough for it
Don't need them normally but mine was quite wide and part of it was to also act as a brace, and help with jigging up etc. Shape and motor comes into it also, vibes etc.