It sounds as if you have mildew/algee on the furniture and you will need to remove it will a solution of 3 parts water 1 part bleach and a couple of onces of liquid cleaner such as simple green. Let this set on the surface for 15 minutes and scrub with a brush to remove all the dirt and rinse well with water. Allow to dry and remove any loose and flaking paint and rust with a wire brush and and prime any bare spots and finish with a spray enamel.