Using highly acidic phosphating solutions that dissolve rust, brush-on phosphating for large-scale steel structures can reduce the cost of rust removal, without the need of a post-treatment rinse. In very low pH medium, the presence of thiourea helps the formation of high-quality conversion films. A hydrogen sulfide acceleration mechanism is proposed, based on cathodic polarization data and previous studies. The formed films are amorphous and contain high water content, therefore a rather long post-treatment desiccation period is required and supported by analyzing the linear polarization resistance values in a desiccation model.