ErnieK
First of all welcome to the forum
You wrote
I have just started sausage making, using a Porkert No 8 mincer. The non tinned surfaces of this mincer leave black marks on a cleaning cloth no matter how well they are cleaned. Is this harmful ?
Not as harmful as possibly offputting if some discoloured meat gets into your casings
You wrote
Will it reduce with use ?
No, its because the grinder plate and blade are typically made from carbon steel. If you like you can purchase a stainless steel plate and blade.
You wrote
Would it be reduced if I coat with vegetable oil after use ?
I would recommend that you DO NOT use vegetable oil, as it could turn rancid if coated and not used for a while. I would suggest that you purchase and use some FOOD Grade silicone grease or spray.
I hope that this information is of some use to you.
kind regards
Parson Snows
PS DO NOT use anything abrasive when cleaning the "tinned" surfaces of the mincer