I've read a bunch of threads concerning this. Many people say that you should alternate between only horizontal and vertical sanding (i.e. not circular or diagonal). Why is this?

 

fyi they sell a headlight repair kit at harbor freight tools for like $10 that works pretty well imo

 

3M has a killer kit also at Kragen/Advance Auto.

 

wait wait...that's not my question...my question is why do instructions and DIY's usually say to sand only horizontally and vertically, with alternating grits? what's wrong with sanding in circles?

 

Circular marring is harder to remove .. FYI I just sand headlights horizontally.

 

really...now i'm curious...why is it harder to remove? just would like to understand the physics of this...

 

It makes no difference! sand any way you want. The only reason you may want to alternate sanding marks when changing grit is so better guage when you have removed the coarser grit with the finer grit. If you still see horiz marks for example when you are finishing up in a vertical motion you know you still have some more sanding to go before you are finished. Otherwise there is really no point and it makes no difference. Some people simply like complicating things

 

When you wet sand clear do you do it circular?