Mine were gunged up with grease by the local 'professionals' (which went solid during a lay up), despite me telling them to oil them. When I found out, I took off the nipples and cleaned them up and then used a cotton bud soaked in thinners to form a void at the top so that I could get some oil in there. Worked it through over about a week. The oil eventually softened the grease and, as Chris said, the muck eventually softened enough to be forced out.