Any filler will work. I use bondo, but it doesn't make a difference. It's taking the time to get it smoothed out right. I will bondo it, use a high speed disc sander with 80 grit to knock off all the high spots. Switch to a finishing sander with 80. Bondo again until all low spots are filled. Finishing sander with 80 or 120 depending on how rough it is. Hit it with a high build primer and usually hit it several times. I will wet sand with about 200 grit here until there are no high or low spots. If you see low spots that are real bad after a coat or two or primer and the primer shows what you missed a little dab of bondo smooth it with the finishing sander and go back to primer until it looks like a baby's ass. If you don't prep right. Your paint is going to look like your grandma's ass. Hit by a bag of nickels. If you are wondering if it's smooth hit with a touch of spray paint just a mist. It will show imperfections. Then hit it with primer and smooth again. Clear coat is not the time to try and smooth body work out.