Gray, ….IMO, automotive putty isn’t fine enough for this hobby,…..for filler work, I use JB Quick Weld. A little tougher to sand, but 10X smoother application than putty ……also, you can’t rely on paper alone to finish. You need to fashion blocks for leveling. I have some blocks less than 1/2 “…….I start with 800 or even 600 if it’s a lot of leveling. Just running paper over filler with your finger tips will leave you with uneven surfaces…..changing directions when running the block also helps…..
Pat