Im not really a body guy but I want to experiment with some things. What grit paper should you use for what?

As an example I have a stock spoiler that is very oxidized. What grit should I use to smooth things out before primer? then after primer? first coat? before clear? The only thing I know is start with 1500 (wet sand) on clear coat and up to 3000 depending on the quality of finish you want.

Any help would be appreciated

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240 before primer and 320 after primer. You shouldn't have to use anything between coats of color and for clear. Use a tach rag after each coat of color, never for clear coats. If you do need to sand between coats use 400-600 grit wet. After clear coats it's up to you but 800 or 1000(wet sanding) followed by a three step buffing process before the clear cures is normal. It's tougher to buff a cleared part after 24 hours(or whatever is stated by the manufacturer).

Now if you are rattle canning this I guess sanding between coats would be best. If you are spraying metallic or pearl, NEVER sand between coats.

Hope that helps some. Good luck.