The alodine is used for corrosion prevention and usually done in a bath but small spot areas can be done with a brush or rag then washed off with water. Alodine is most commonly used in aircraft manufacturing and a surface that has been worked say counter sunk rivet holes would be alodined before the rivets are installed. I first started my career in aircraft manufacturing and always had a small amount of alodine and a brush on the bench. As the body panels on our Landovers are aluminum it makes sense for them to be alodined as it provides very good corrosion resistance.
1971 series 2a 88, series 3 trans, Fairey OD, owned since 1978.