What grit sandpaper do you guys suggest I use to rough up the fiberglass gelcoat for primer?

This is for race motorcycle fiberglass fairings.

Thanks!

P400 dry by hand or you can go one step coarser if sanding with a DA and then using a 2K primer/filler. P600 by hand or P500 on a DA if you're just using a sealer or a 1K primer. Make sure the entire surface is dulled whatever you use.

A lot of people will recommend much coarser grits under primer, but these are more than sufficient to give good mechanical adhesion and not too coarse as to risk scratching being able to be seen after the primer dries and shrinks.

Chris, isn't kidding when talking about avoiding rougher grits. You just need "enough" surface profile to grab your overcoating process. We've had guys on here in the past that roughed up the gel coat so much they had to go with high build primer to get over their nasty scratches....

Awesome thanks guys! Going to use a 2k filler primer so P400 by hand it is. Thanks!