I use just a standard curved Exacto blade. I used to use a ceramic tile for the cutting board, but switched to a piece of 1/2" thick UHMW plastic.
I cut the sprue close to the waste part of the fret, then trim it close with one of two Xuron shears designed for cutting brass or stainless steel PE, # 9180 (Longer), # 440 (shorter). Then, with a swipe or two from a diamond hone (bought at a fishing supply place, used for sharpening fish hooks I think), lightly dress the edge.
Just takes a bit of patience.
EJ
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" 1/700Hasegawa 45
" 1/700Dragon 44
" 1/700Trumpeter 43
" 1/540Revell vintage 62
" 1/350Trumpeter 42
Fair Winds and Following Seas