After trying every solvent and cleaner I could get my hands on I gave up and attacked it with a flap wheel and wire wheel. I'd only used a grinder once before to chase out a couple of electrical sockets and it was so messy I never tried again. I can't believe how much I've been missing out though, everything is going to get a good grinding from now on :-)

The angle brackets were probably the worst and they were awkward because as were small. Here's a before and after shot.
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So after a few hours with the grinder I'd turned a pile of oily steel into a pile of clean steel ready for the next step.
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From reading around on welding forms it seems the next step is to pickle with phosphoric acid rust remover and then undercoat. Does this sound right? I want to get an undercoat on before starting construction as I think this build will take a while and I don't want all this work being undone. If I decide to get a welder it should be fairly easy to wire wheel off the undercoat in a few places. Any recommendations for undercoat? I was just going to pop down Halfords and get some car spray undercoat.