You can use resin to seal the edges, this will allow anything you use as filler to adhere to the side of the mdf. Unless you seal the wood first it will release from the wood and you will have lots of paint issues. You can also use superglue(i like 2 part) and a bondo spreader to get a even coat. After you seal the edges of the wood you will be good to go with bondo, duraglass, slicksand, etc. Hope this helps in the future.I was thinking of bondo but at this point I not sure if it will do. What about fiber glass resin?