that yellowish film you see probably is part of the healing process. as the body heals a wound it frequently produces an overabundance of proteins. these proteins often look exactly like your description: a yellowish film. this film often is described as thick sticky or gunky. as long as the area of the boil doesn't become extremely painful red or hot and as long as you don't start running a fever the healing process is likely proceeding normally. if you experience any of the above symptoms you should contact your primary care provider. you can gently cleanse the area with lukewarm water and mild soap. you don't need to scrub off the yellowish film. hope this helps!
should the boil form a yellowish film after drainage