One-side razor blades, carefully by hand or use a small handle/holder. Spray on decal remover as the lube to get the blade under the edges. Sometimes you have to wiggle/saw the blade to get a decent bite under the decal. Heat gun can help sometimes. I would suggest staying away from WD40 or PB Blaster. While they are good decal removers, you don't want petroleum contaminants all over a surface you are about to paint.
You may gouge the paint in a few spots, just sand those out before primer.
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