body filler help....

[gildardo01]

im having some trouble on a 75 c10 stepside.. the problem is the body filler is cracking i know its not because of the road vibration or anything like that because the truck doesnīt even run yet... the previous repair on the truck (not by me) applied about 1/4 to 1/2 in of body filler on the rear fenders and on the hood...
my first idea was to remove the crack with a wire cup brush on a grinder... and just refill it with body filler... problem is that right where the new filler meets the old filler thereīs a crack... so i removed the cracks and a much wider area this time, but i dont know if i should just refill them or what... i canīt buy new fenders and removing ALL the body filler on the whole fender or hood and getting the dents out with hammer and dolly would be wayyyy to time consuming... any ideas? i think it might be from surface rust on the metal that is emitting gas and it is just trying to find a way out... i thought if i remove the crack and the wide area, clean the surface metal real good then add primer and then add body filler that might work... havenīt tried it but do you think it will work?

[jaredts]

What are you prepping it with?  Solvents will soak into the existing filler and cause problems.  Are you carving it out and filling a big crater like in the picture?  From the picture it looks like the filler is way too thick, and also appears to have been added all in one thick layer.  This will cause cracking.  I think you're asking for trouble if you don't grind it all off and try to do some metalwork to eliminate some of the thickness.  I don't recommend it in this case, but if you are determined to try and fix what you have you should feather the edges of the area instead of grinding straight down into a hole.  Apply filler in thin layers.  Consider stepping up to a glass reinforced filler.  Never put solvents on filler.  I'm not the pro around here so maybe someone else has some thoughts.

[gildardo01]

ii didnīt add the filler you see in the picture... that was there... and the first time i did it i just carved into it... i didnīt think about feathering the edges... i do know you have to add thin layers of filler at a time... kinda like when you paint... you use thin coats to let the fumes and everything come out... i havent gotten to a point of where i have to use anything besides proxy primer and body filler... after i get everything done with body filler iīll shoot the primer then add plastec i think thats how you write it... its like this primer looking think paste you add with a rubber spreader then sand it down, after that with everything is smooth and straight iīll use the universal primer...

[klaussk]

Honestly I think you should remove the body filler and try to knock out as much of the dent as possible. I don't think it'll take very long on the fenders. You will never get get them completely flat again because the metal is streched, unless u have a press and mold. A half inch of bondo is leaning on the overly thick side of things and will eventualy fail. I suggest taking the time to do a better job on it and not having to do it again. Use a small hammer with a flat and blemish free surface and a dollie and just beat it as close as you can get it. Then fill what is left. Make sure you sand the area to be filled with some 40 grit and then clean it with paint thinner. Then use  the bondo.

[Blazin]

1/4" is to thick for body filler.  I never prime under filler either.

[bake74]

Honestly I think you should remove the body filler and try to knock out as much of the dent as possible. I don't think it'll take very long on the fenders. You will never get get them completely flat again because the metal is streched, unless u have a press and mold. A half inch of bondo is leaning on the overly thick side of things and will eventualy fail. I suggest taking the time to do a better job on it and not having to do it again. Use a small hammer with a flat and blemish free surface and a dollie and just beat it as close as you can get it. Then fill what is left. Make sure you sand the area to be filled with some 40 grit and then clean it with paint thinner. Then use  the bondo.

1/4" is to thick for body filler.  I never prime under filler either.

     I could not agree more.  Knock some of the dent out.  The less body filler the better when doing body work.

[jaredts]

You might even find that if you grind all of that filler off, it didn't need that much to begin with.

[mattgood]

1/4" is to thick for body filler.  I never prime under filler either.

I also agree... It will just crack again if you use that much to fill the gap. I would take the extra time to strip the whole fender, or get a new one.