Hit the rust with a rust converter (from Bunnings ) and let that dry over night.
Next up I gave it 3 prime coats with kill rust (in a tin), which turned out quite well.
Finally (paint wise), just used a old tin of touch up paint from our old Falcon, (wasn't even sure how close the colour was) gave it 5 light coats and the result was this.
Of course I sprayed as much fish oil as possible inside the tunnel to hopefully slow any further rust (god forbid it come back!)
Then just put some "Knead it" to fill the hole and give it a bit of water resistance.
This hopfully won't be permanent, just until I can afford to get new metal welded in there properly.
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