refluxing some dcm in the flask works extremely well to remove that tar. or if you wanna be lazy, put some dcm in the flask and run some hot water over the flask until the dcm starts boiling and swirl it around a bit. and if you don't wanna use your solvent dcm, you can get dcm containing (>80%) paint and adhesive removers from just about any hardware store for pretty cheap.
once your washing techniques start to improve, you won't get much of that tarry shit. swim considers h2o washes to be essential after washing with any acid or alkaline solution. and don't just stop with one water wash. perform h2o washes until the ph of the aqueous phase is near neutral.