Cleaning Out XJ6 Fuel Tank Rust

If the flakes of rust are THAT big then its a surprise that the tank doesnt leak

After the rust is removed, I'm worried it might. I've poked around suspicious spots on the outside and it seems OK, but apparantly you can get tiny pin-holes that are otherwise plugged up by the rust.

Or the gunk. I spent some hours today painfully trying to scrape out the thick sticky black gunk that I thought I'd already drained out after softening it with laquer thinners, but there was plenty left disguised as metal by a layer of rust dust. The car was sitting for unknown years and it must have distilled from the old petrol. At least it does seem to have protected the tank surface underneath in places where it pooled, but it glues the insides of fuel pumps together too.

After more research I think I'll go for the washing soda and battery charger "electrolytic" rust removal method. The acid would eat away some of the metal too, which might leave a surface more prone to rusting again. Or so I gather...

I couldn't find a way to mount it on my cement mixer, so I used a reel for winding up wire instead and turned it by hand for 15-30min each day for a few days.

No sign of leaks yet but trying to wash the rusty sludge out turned out to be a major effort which took days with filling/draining and spraying water, trying to dry out, sloshing lacquer thinner around, blowing out with compressed air. By the time I got the sludge mostly washed out, the moisture (in spite of my drying efforts) had caused surface rust to return to most of the areas which had been clear. It seems like starting again with the derusting at that point would turn into a lifelong occupation so I'm going to refit the tank and hope that once sealed up properly it won't keep rusting enough to make the larger rust flakes which clog up the fuel pump. I assume the acid turns into a similar sludge?

In addition to the rusty sludge it turns out I missed plenty more of that black gunk which was in a big mat mixed with rust after derusting and I had to spend more hours picking bits out the drain hole with a wire. I think I'll have nightmares about black gunk. Trouble was it looks just like the metal of the tank once there's a layer of rust dust stuck to it.

The method does work fairly well and is very cheap. I read 10g washing soda per litre of water, but the conductivity was so high with 200L in the tank that I had the fuel tank in that it was maxing out the current from my battery charger, so I had to connect a high-wattage 1ohm resistor in series to avoid overloading. I left it for a couple of days and it seemed to work. It worked well at breaking off rust flakes with the bubbles, but that was pretty frustrating to have the tank full of flakes of rust again after so long spinning it with the chain then blowing it out thoroughly with compressed air. Maybe all that preparation is a bit pointless? Converting the surface rust to the black derusted finish worked better on the side facing up than the side facing down (with the tank laying flat). I couldn't turn it over though since the rounded side of the tank would trap an air bubble.

The main issue is that final step of cleaning off the gunk, which is trivial for the solid parts cleaned in the online guides, but for a tank you can't get your hand inside to scrub out manually, it's surprisingly difficult. Lots of brown stuff washes out, but still leaves plenty stuck to the sides which you see when you scrape something over them. I don't know if there's a better liquid than water or lacquer thinner for washing it out? I guess if I could bake it dry, but I only have a little electric heater I can poke in, then blow it out with compressed air, and I'm not sure if the prolonged warmth and damp air actually encourages re-rusting before it dries (and you can't really tell when it's dry inside anyway). Of course it doesn't help that the weather is also damp here in Australia at the moment.

Overall I think my experience makes a good case for buying the repro tanks. But since I've got things set up and learnt some lessons I might still give the other fuel tank a try.