How to remove paint from grooves less than half an inch wide?

My bathroom ceiling had mold coming through it. There looks like there are at least 2 coats of paint on it. I have used a heat gun to get rid of alot of the paint but the ceiling has small gooves running along the width of it. It's kind of like slats with the gooves between the slats (if that makes sense). Anyway, the gooves a very narrow (less than half an inch) and difficult to get the paint out of. There is no scraper tool small enough to fit. I have tried a flat end screw driver but that has not been very successful. Any ideas what I can try? I have used a paint stripper but that did nothing. I have also tried sandpaper but that does not seem to remove the paint either.