i just got in from filing 4 year old bondo on the right front fender of my '75 k5. the flat and round file 10" blades are both worn down after about 20 minutes each. now they kind of shine the bondo instead of grinding it off in long curled shavings. the file blades are pretty low end, "bondo" brand i got at the local shuck's auto supply. is my problem the bondo being old and hard or the blades being cheap? maybe there are some hardened steel type file blades i should seek out at a body shop supply joint. maybe i should get some sanding blocks and go with 120 grit or so instead? i fear that i will just clog up the sand paper with bondo after only a few minutes tho... pointers anyone?