Whereas DWM in Berlin rust blued their Lugers, Mauser used an acid solution. Mauser used up a large stock of DWM parts when they took over Luger production. So if the gun was rust blued, you could very likely repeat the process by carding light rust off with a fine steel wire brush. Of it´s cold blued, a specialist would must likely have to remove the surface bluing. As stated previously, it´s your choice whether to maintain the gun´s collector value, or to achieve an attractive family heirloom. As far as I know, the specialists on the Luger sites are booked up for months ahead and I would personally ask them to for their advice.