Are your stripes inlayed or are they on top of the white ? I hope they were inlayed ..
If they are not inlayed then you should sand and buff ALL of your white..then tape and prep for your stripes..then sand and buff out your stripes and Try to feather in your tape line without burning through the clear.
180 is way to much for any gel finish first off.. 220 is max .. 320 is standard for heavy orange peel starter grit..
Basically .. if your inlaying your gel then dont sand anything until your done with the spraying..
If your Overlaying stripes..then yes you have to buff and polish your Main surface First before you do the stripes or add-ons.
If your Overlaying its faster to do the sand/buff stage Through the stripe area Before you lay out your lines for the stripes. .. Layout your stripe tape and reprep to the tape line for your next coatings..
This is why Inlays are Faster and Sooo much better
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YD.