you could sand it off but itd more than lilkely get hot and furr up, best to sand it all down with like 320 and use some high build primer to build it up, then wetflat it down with P800/1000 or 1200 to smooth it
as for filling it, make sure you really rough it up well all inside the line or the filler wont stick at all. id be tempted to p180 it, all inside, all along the line, and get some fibreglass in there, run your finger right round to push it all in and take off any excess, sand that down then fill over it
high build primer is just primer with good covering potential, usually 2k (mixed with hardener) but it can come in aerosol form. personally i dont notice much difference between that and regular cellulose in the aerosols.