What you are seeing is some residual salts from the phosphating process. This happens occasionally on very rusted and porous surfaces, it is a byproduct of the rust being converted. In most situations, this can be simply wiped off before applying a top coat. If you use a mild detergent to aid in this process, make sure to rinse thoroughly and let it fully dry before applying the top coat. If you are not applying a top coat, this residue will slough off on its own.
After letting the XionLab Rust Converter dry, a dull white residue appeared on the surface. Is it normal?