Do NOT use denatured alcohol. The added chemistry forms a colloidal suspension when mixed with water (among other nasty things) and will harm the emulsion.

As for soaking the plate, this was done in early days to sensitize, but was time consuming and not very precise. The amount to use is quite debatable, and my friends could not even begin to suggest a starting point. The soaking would have to be by trial and error and then, how would you quantify it? This is an added difficult step, verifying the effectiveness of the sensitization step.