fiberglass

you can use feather fill. just go thru and spray a few medium coats on it. the first coat will soak in the mdf. the second will sit on top pretty well. then go thru and sand it with 80 grit if there are any runs. if not you can start with 120 or maybe even 220. sand it to 320 and shoot a coat of sealer primer over the top. this will give you a uniform color under the base coat. you can spray on the sealer primer pretty light. sand with a 400 grit and maybe a 600 grit. then shoot your paint. if you are gonna spray it in a place where there might be a lot of dust in it, wipe it down with a tack cloth in between the base coat and just before you apply the clear.