starch does not normally build up on an iron- fusables on the other hand can make quite a mess- always use a pressing cloth when using fusables & you will avoid this problem.
a pressing cloth can be any lightweight piece of fabric- muslin works great- parchment paper also works well...you place it over the top of the item you are pressing down that has fusable on it- then press- any excess fusable will stick to the pressing cloth and stay off your iron.
a hot iron cleaner is an item you may want to keep on hand.