Hi Graham, stay with mirror on top while using the #80 grit.What is your target sagitta? What progression of grits are you using? I used the following for my larger (18" to 25") mirrors: #46,#80,#120,#220,#400,#800 Silicon carbide (for all). With smaller mirrors I wouldnt use #46 (but when having target sags of 6-8+mm the courser grit saves time in the initial hogging out).
