I have messed around with a variety of mustard patina application approaches. I find that I achieve the best results with multiple coats, each followed by a very light rubbing of the area with a scotch brite pad. It helps to create a more even finish. For a neat look, you may also try dabbing on the mustard with a small sponge or Q-tip. It gives a mottled look. I try to spread the mustard as evenly as possible, as varying the coat leads to variations in the rate and extent of oxidization - which could be a good thing or a bad thing, depending on the effect you're trying to achieve.
Finally, and most importantly, mustard patina is best applied while eating good German sausages. That way you can alternate between applying mustard to the knife, and enjoying some yourself.
Nice looking Nessmuk, by the way.
All the best,
- Mike