0.35 mm is too thick IMHO, and if it is a soft material, a regular tightening may be difficult to keep valve chest and valve rod parallel to the cylinder axis.
a simple oily paper will do it or a smear with thick oil based paint.
Be careful with paint or silicon compound to not over do it, blobs of product inside steam chest may hinder valve movements.
but before the suggestion above by Whiskey is mandatory...