On my gypsy thirteen two I have tried replaced the front panel with a Bunnings sheet and two steam mops, that broke straight away, so I tried some better quality 4mm ply and soaked it with boiling water over a stand with ratchet straps to try and get a bit of shape before applying it, I managed to get all the nails on but everything looked out of shape and I had a few cracks at the bottom so now I'm thinking of going bendy ply before the frame turns to more nail holes than wood. I made some calls and I can buy a 3 mm sheet of a super strong bendy ply for $125, or a 7mm sheet for around $90.
3mm does seem a little thinand expensive but they tell me it's a strong highly engineered ply used in aircraft. 7mm just seems thick and heavy.
Should I keep searching for other alternatives or go with the 3mm?
From what I've seen, most bondwood caravans would have used 3-4mm ply in their construction. Many people who renovate them these days tend to think that thicker is better, and mostly it's really a case of thicker is unnecessary - besides adding to the weight! I read the advertisement blurb for your van on here vintagecaravans.proboards.com/thread/2013 and notice that they too refer to using a "waterproof ply as used in aircraft construction" on the Gypsy 13.2. So obviously it would not have been very thick (my guess probably 3mm at best).
Not sure if you have already done this, but sometimes it's better to go direct to the specialists when talking about a particular subject. Members of forums such as this generally know a little about a lot of things (I include myself in this), but when it comes to specifics such as what you're asking, I would go to the manufacturers of plywood, rather than retailers, for advice. I have found Austral Ply to be very helpful in getting advice. Their website is useful and their technical people are very helpful. The added advantage is that they use Australian timber and manufacture their ply here, so you know what you're buying. They don't sell direct to the public but will advise you where in your state you can get their products. Their Exterior ply has a 3mm thickness which should bend for you. It has an A/C facing and A class bond (guaranteed for 25 years).
I'm intrigued when you say that everything looked out of shape. Is the framing actually even? Also, are you gluing the sheets on as well as nailing? If you are gluing with something like liquid nails, the nails will act as a tack while the glue dries and then the glue will take over and fixing and shouldn't pop the ply.
3mm looks very thin but they say it is a highly engineered wood that is extra strong, I am trying to keep the weight as close to original or lighter if I can get it. I used selleys one construction adhesive and nails but it bulges out between the nails at the top and bottom and bows in between the bracing in the center at the top and bulges at in the center at the bottom. The original wood looks to be 4-5 mm thick. The caravan does seem to have a few less conventional features in it's construction, the chassis is a space frame kind of chassis like some sunliners had with the z bracing between a top and bottom tube, the van is not lined on the sides but the front and rear are lined.
It sounds like you may have spaced your tacking nails too far apart. Perhaps also you started from the edge inwards rather than the middle outwards on the side to side attaching. That's about the only explanation I have.
Ok, finally got a price for bendy ply 5mm, $396.50 for one sheet, that is from austral, their Newcastle dealer is ntt, ntt can't sell to public so I'd has to go to nhs, so by time it arrives at our local store it's $396 + delivery. So it looks like I will have to go with the 3mm. Just not sure if 3mm will seal the same at the edges under the aluminum strip