UKH

I typically use 100-110g gas canisters and I got the tool to refill them from a larger canister. It’s much cheaper this way and I don’t need to worry about lugging half full canister. 

However, even with drying them out after coming back home, the canisters tend to rust, which made me rethink how many times it would be safe to reuse it until it’s time to retire them. I really don’t want it to fail, leak or explode! I witnessed one being set on fire and it’s not a pleasant feeling, even if we got it under control quickly enough. 
 

I did clean one as much as I could with isopropyl alcohol and WD40, but there’s still some rust left.

An amusing thread juxto has just occured -

Rusty gas canisters

Huge explosion in Beirut

To the OP, it just looks like surface rust and probably isn't that bad but if you're worried then maybe just get a few new canisters, they're not super-expensive after all. Would painting them with Hammerite etc before use be a good idea?

That can wouldn't worry me whatsoever.

Yeah I have a few that I rotate (read: whatever feels least empty since last batch refill). 
 

I have 3 small canisters and few big ones (450g I think?). This means I can just keep topping them up without having to buy small ones as frequently. 

That's just surface rust, i think it would take years to rust through the steel on a canister to the point of failure.

Thin material intended for single use? 

Having seen the remains of an (in test) dive cylinder that exploded whilst being filled I'd err on the side of caution. 

I can't remember the last time I had a gas that didn't rust on the bottom as soon as you leave it on wet grass for longer than about seven seconds. I tend to buy 500g canisters and am pretty frugal with gas, and some of mine have become much worse than that. Never had one fail, and I don't think it's a danger unless it looks really quite knackered.

That’s interesting. What psi do you fill a dive tank to? 
 

On a separate note how would that compare to the pressure in a camping gas cylinder? 

A standard dive cylinder would be rated to 232 bar (only Americans use psi in diving). 

Slightly different situation. Butane liquifies at 32psi (at 20C), compared to 200bar for a dive cylinder.

Although being cautious sounds sensible!

So that’s 32psi v 2900psi. Quite a difference! 

I had 1 filled to 300 bar only yesterday

The level of rust in the photo is not an issue. The biggest problem is usually rust preventing a good seal to the burner. I would expect that to be an issue long before the can's integrity was compromised. 

I've actually got quite a few cans in worse condition than that.

If you plan to keep re-using one and it's worrying you why not paint a fresh one to keep the rust at bay. Inside should be oxygen free so it's only the external part that's a big worry.

How do you ensure they're not overfilled, do you weigh them? Rust will most likely lead to a pinhole leak (bad in a cupboard) but overfilling and heat could lead to a burst (deadly in a tent) since these presumably don't have a pressure relief valve which would at least give a (fiery) warning.

edit: butane/propane mix vapour pressure roughly doubles from uncomfortably cold to too hot to touch so bursting is probably not much of a worry in reality.

jk

I didn’t know you could refill them. Do you have a link to the tool needed?

A fairly comprehensive article on it here. I've not tried myself. 

https://adventuresinstoving.blogspot.com/2012/01/gas-canister-refilling.htm...

Cheers, having read it I’m not convinced it’s worth the time/investment/or potential risk. My half full canisters go in the back of the Van for a post sea swim hot choc!