I'm looking at buying a container of oxalic acid crystals and some distilled water to make up a solution for rust removal. Does anyone have any ideas or suggestions about the concentrations to use?

Ben

Oxalic is a very mild acid even when you make a fully saturated solution - remember those from school? Stir in as much powder as you can until no more dissolves. If you warm it up a bit it will take more. I don't bother with gloves when using it for cleaning teak furniture. It will remove rust stains from cloth and wood, but it's not much use for rusty steel. Hydrochloric is about best for that, but do wear full protection.

Don't use acid on any bolts/screws, it weakens high tensile steels very badly. Best just to wire brush or replace.