rhys to be honest mate any frame that has been chemicaly striped wil have the same problem,
basicaly, when you dip a frame the whole inside of the tube are flooded therefore cleaning them
and gegreasing them, it's just part of the process that has to be done,
i really wouldn't worry about it, blow out any loose bits with an airline and tbh it will take years to
eat through from the inside, if it ever will, think you will find it just loose surface rust and nothing else,
also could be residue from the acid tank that has now dried out
