Im doing a little filler work om my 87 runner today. Last night I sanding off the old crappy primer that the PO had spryed on it. After I got that all taken off I noticed that there was a little bit of old body filler there (No big deal not alot) but there is a new dent about the size of a dime or so kinda close the the old filler. Should I sand all the old filler out or should I put my filler over the old?

 

As long as the old filler looks ok, it is fine to wipe over the new spot, BUT I would recommend wiping over the whole area and block it out as one area. If you focus on just the small dent, you will probably find the old area affected by the new work.

 

If I was doing it I would look closely at the old filler and make sure it isn't cracked, or buckled in any way. If the old filler looks like it has a good bond rough it up with 80grit paper including out to (and around) the dime sized dent. The dime sized dent should be within a reasonable proximity to the old area. Roughing up the area will make an even bonding surface for the new filler on the whole area needing work. Spread on the new filler (over the entire area) and continue the body work.
YODAFORCE is correct in the post above. You are trying to create an illusion. The illusion being no more dent and no visible bodywork. Working little dents within the area of a big dent saves sanding and it is easier to create an illusion of perfection.

 

what i do if there is old filler sand it out just to make sure that someone didn't fill a rust hole or whatever. i have taken whole Lincoln quarters down to metal and found some neat tricks, like filling huge holes with screen and filler, neat. I'm always grinding out old filler. i don't care if it looks fine you just never know whats behind it. most of the time if you have a good hammer and dolly set you can metal finish better than what others have filled up and only need a light skim coat of filler and a quick block to get it flat.