Bondo not hardening?

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PostPosted: Wed May 26, 2010 8:07 am
Using some bondo about 2 months old it wouldn't harden. It hardened to a degree but I could still dig into it with my nail. I went back to the store and got another can of bondo. Same thing happened. Went back and got ANOTHER one and the same thing happened. ( I did many tests to confirm that the mixture was right and with different hardener tubes to make sure it wasn't the hardener)

I remember bondo hardening ROCK HARD after 15 minutes. This stuff is kinda soft and tacky on top and then when I sand it it's harder underneath but still I can dig my nail into it with a bit of pressure. Am I going crazy here or should I not be able to dig a fingernail into it.


( it's 2 part filler 3m )

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PostPosted: Wed May 26, 2010 8:19 am
It sounds like you are not putting enough hardener in it, not mixing it up good enough, or its just a bad batch of bondo..............???

If it were me I would add more hardener.
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PostPosted: Wed May 26, 2010 10:10 am
Well there should be a slight 'skin' on top that is kinda rubbery on cured filler. You have to break through this skin to get to the more solid filler. Usually I wait until the filler starts kicking off and quickly shape it by hand with 36-60 grit....then I wait for it to cure and buzz over it with P80 on a DA. Then I block by hand with P80-P180 before primer.

The 1/8" ribbon of hardener across a 4" pile of filler is a good rule of thumb. Even slightly less than that in hot temps. You'll have to find what works best for your temp and humidity.



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PostPosted: Wed Nov 16, 2011 10:40 am
Having similar problem...or maybe different. Working with US Chemical Fiberglass Body Filler. Relatively thin coat on a couple of places where there was pitting rust on the frame. Rust was removed primarily by grinding then I washed the area with Fast Etch and wiped off with water per instructions. Area was fully dry. Mixed small amount of the filler as it has been years since I last did any body work.

Spread the filler on frame in a couple of spots. I am sure I got enough hardener as it was just starting to get stiff as I cleaned off the mixing board. It was cool in garage 60 - 65 so I hit the filler with warm air gun and got the metal in the area warm to touch. est 80 The filler was firm but tacky to touch so I left it cure overnight. again it was cool but I figured it had kicked and if I gave it 10 - 12 hours it should be fine.

Next day tried to do rough sanding w/80 grit but the paper loaded up real fast. Surface of filler was easily scratched with fingernail but once the surface was removed the filler was firm and hard. I could scrape the surface and get soft filler to come off. Maybe 1/64" worth then the filler seemed normal hardness for cured filler.

The suggestions to remove the soft surface w/80 grit don't seem to be working as I am quickly loading the paper. I am using up a 6" disk to clean off a couple of square inches of filler and it isn't clost to being clean enough to go to finer grit paper. The filler did cure up as there have been no areas where it pealed off the frame or balled up. I am just having problems with the surface of the filler cloging the paper I am using..

If i did something wrong and the filler really isn't cured I would rather clog a wire brush and remove it than keep working and end up painting over bad filler.

I definately need help and advice. I can post some pictures if that would help.
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PostPosted: Wed Nov 16, 2011 10:59 am
Most fillers skin over quickly and leave a top layer that is a little soft; the solvents escape very quickly from that top layer but underneath you'll get a normal cure. I think you'll be ok if the sub-surface is hard.

1/64" is a little much -- usually it's a thinner film. I'd scrape it as much as possible and/or use a metal rasp / surform file.



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PostPosted: Wed Nov 16, 2011 11:36 am
I bought this stuff about 5 years ago, it was about $12 for the gallon at the time. Just out of curiosity, I bought some fresh bondo hardener for it to do some work in the engine compartment, and it gets rock hard. PITA to sand, so I don't use it on outside panels. However, when mixed 3:1 with any 2K glazing putty, it's not so bad.

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PostPosted: Wed Nov 16, 2011 11:47 am
It's usually not the filler that's the problem but the hardener itself. Most of the time the stuff is out of date as it goes from warehouse to final retailer. I've never had problems with hardener from my local paint jobber (high volume supplier) but have gotten bad stuff when I needed something in a pinch from a local retailer.
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PostPosted: Wed Nov 16, 2011 1:25 pm
chris wrote:Most fillers skin over quickly and leave a top layer that is a little soft; the solvents escape very quickly from that top layer but underneath you'll get a normal cure. I think you'll be ok if the sub-surface is hard.

1/64" is a little much -- usually it's a thinner film. I'd scrape it as much as possible and/or use a metal rasp / surform file.



1/64" may be overstating but it is more than just thin surface.

Would laq thinner remove the tacky stuff or would that also attack the cured filler.

Just picked up a new cheese grater rasp and more 80 grit at HF. Will have to use the cheep HF paper arount front suspension mounts as not enough flat for the rasp. Will also try more heat.
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PostPosted: Thu Nov 17, 2011 10:24 pm
The only way to remove the filler is to sand it, wire wheel it, or even use heat and scrape it off. I wouldn't wipe it with any lacquer thinner or acetone, it tends to make the filler a little brittle and can smear it if its still soft...

If you think the filler still feels too soft or rubbery, I would remove all of it. Then use some new hardener in the filler or a new batch of filler and hardener to eliminate any problems. Try using a little more hardener since its cold out, 3%+.



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PostPosted: Fri Nov 18, 2011 2:10 pm
Went through the same thing last weekend. Picked up a gallon of Dynatron Gold from Advanced Auto (being cheap) - got warm, set up fairly firm but stayed tacky on surface, never got as hard as the leftover Rage I was using. Tried some more hardener in a test scoop - harder but still tacky and could stick a thumbnail in it. Called 3M : "give it 2-3 hours". Gave it 10-12, nope. Took it back and spent the extra on a gal of Z-Grip at Carquest. This is the second time I've had a problem with Bondo/Dynatron 3M - no more. I won't spend 80.00 on a gallon of filler, but I will NEVER buy the "cheap" stuff without reading a few forums (not that Z-grip is "cheap" , just less expensive with a real good reputation. Good sandpaper as well.
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