the courser the scratches the better adhesion. sanding (just knock off the shine, try not to go all the way to the metal) everyone I have done I sanded with 180g, everywhere except bed floor and wheel wells and I used 80g

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1999 MAZDA 2500SE, 2.5L 4-clanger, 5 speed, Green n slow. Rangers as daily drivers since 1988.
ASE Certified Mechanic, Builder of Show Destroying Protouring and Classic cars..