If you click on one of the photos you can see the whole bike, a Katakura Silk.

Most of the OA treatment was on components which were soaked in a solution. Rust stains on the frame, as in the picture, were treated with tissue paper wrapped on the affected part and then wet with a brush. We kept wetting the paper with more solution until it had done its job.

You have to neutralise the Oxalyc acid because it's an acid and will keep eating after the rust is gone. OA makes a bicarbonate solution fizz. When the fizzing stops, the OA has been neutralised.

If I had a frame with a lot of rust, I'd probably try to find something to put the whole frame in, like an inflatable paddling pool, like someone else suggested, I think.