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Having Trouble With Bondo - Need Advice


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#1 JDBrian OFFLINE  

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Posted September 21, 2012 - 07:35 PM

I am doing a bit of body work on the MF8 fender deck and hood and I have trouble getting the Bondo sanded. It seems to be just slightly sticky and loads up the sandpaper. Am I not using enough hardener or too much maybe? Anyone else dealt with this?

Thanks

#2 chris m OFFLINE  

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Posted September 21, 2012 - 07:44 PM

Was it humid when you applied it? or did you put it on real thick?

But I do know if you don't get the hardener amount correct it will cause this to.

#3 olcowhand OFFLINE  

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Posted September 21, 2012 - 07:47 PM

Probably too little hardener, and maybe needs more cure time. No matter what, the outside "skin" is what clogs paper. If thick enough, you can lightly go over the top layer with a wire brush, then go to paper. The wire brush will remove the skin layer.

#4 Cvans ONLINE  

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Posted September 21, 2012 - 08:10 PM

They make a tool called a cheese greater. Use that first to take off the high spots and do your rough shaping and then use the sand paper.You can use a heat lamp to speed up the cure time if you used too little hardener but just warm the bondo don't over heat it. The cheese greater works best when the bondo is just a little rubbery.

#5 CASENUT OFFLINE  

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Posted September 21, 2012 - 08:18 PM

A little bit of hardener goes a long way...just make sure the filler is uniform in color...if you see patterns in your filler swirls or an uneven color scrape it out and start over. The trick is not to 'stir' it just use pressure to fold the hardener in, this will also keep out air bubbles. Filler normally takes 20-30 minutes to set up. The stuff is cheap so don't hesitate to 'redo' it if necessary. When I first started doing paint repairs I treated it like it was gold and never wanted to do a 'do over' but, now that I have the technique down if there was ever an issue I would not hesitate ...I also recommend going with a 'better' brand than 'bondo' ...evercoat makes several higher end fillers...and use featherrite is a good 'low end' filler. The better the filler the easier it sets up and easier to sand. I also like to use European Filler...it is I think $35 a gallon and sands pretty easy...

One note this guy doesn't do is to knead the hardener...make sure you take the air out of the hardener tube and spend 1-2 minutes to mix it up. once it sits for longer than 30 mins...the chemicals in the hardener begin to seperate...


#6 olcowhand OFFLINE  

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Posted September 21, 2012 - 08:31 PM

They make a tool called a cheese greater. Use that first to take off the high spots and do your rough shaping and then use the sand paper.You can use a heat lamp to speed up the cure time if you used too little hardener but just warm the bondo don't over heat it. The cheese greater works best when the bondo is just a little rubbery.


Yes, I used to have a "cheese grater", but lost it somewhere along the line. They work great, but in a pinch, a wire brush helps a lot.

#7 IamSherwood OFFLINE  

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Posted September 21, 2012 - 08:41 PM

Brian, try taking a heat gun to it, to make it set up. I think you where a tad bit shy
on the hardener. As Nutcase said, It's cheap. if it fails on you, scrape it off and redo.

#8 caseguy OFFLINE  

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Posted September 21, 2012 - 09:09 PM

Sounds like some good advice going around so far Brian. The only topic that wasn't covered is surface prep. Before you apply the filler, the surface must be a bit on the roughed up side and also clean as a whistle just as if you were going to paint. I actually prefer to use my body file (cheese grater) when the filler is on the less rubbery side of "green" (still soft, but well on it's way to cured. I tend to get carried away with the removal if it's too soft :blush2: ) I also concur with the diagnosis of too little hardner in the mix. You can apply heat to speed it up as eveyone else has said, or just be patient and it will likely be ready in 24 hours or so...if not, time to start over. Good luck with the project!

#9 johndeereelfman OFFLINE  

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Posted September 21, 2012 - 09:30 PM

I have a question for you guys. Who applies the body filler before priming, and who applies the body filler after priming? Which way gives better results? How many guys don't use body filler at all, and just fill miscellaneous pitting with built up primer?

#10 olcowhand OFFLINE  

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Posted September 21, 2012 - 09:52 PM

You want to apply filler to bare metal. I also rough the metal with an angle grinder to add "tooth".
  • johndeereelfman said thank you

#11 CASENUT OFFLINE  

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Posted September 21, 2012 - 10:17 PM

I have a question for you guys. Who applies the body filler before priming, and who applies the body filler after priming? Which way gives better results? How many guys don't use body filler at all, and just fill miscellaneous pitting with built up primer?


Oldcowhand and I musta went to the same school. I will advise this though. If you get metal blasted or blast yourself and do not have time to do all of the body work in one 'fell swoop' ...cover the piece with epoxy primer or and etch primer to protect it from rusting while you complete body work. Then as you approach one 'dent' or one rust patch completely grind the epoxy primer out down to bare metal then apply the filler. The chemicals in the filler will not 'chemically adhere' to the epoxy or etch so get it out of the way for your filler operation. then you can 'cover' the repaired section with epoxy. This will give a nice smooth uniform color that will be forever protected from 'rrrrust'... then if you let it cure you can scuff it with some '600' or so before you either 2k primer or topcoat it...Good Luck
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#12 Bmerf OFFLINE  

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Posted September 21, 2012 - 10:24 PM

One note this guy doesn't do is to knead the hardener...make sure you take the air out of the hardener tube and spend 1-2 minutes to mix it up. once it sits for longer than 30 mins...the chemicals in the hardener begin to seperate...

I am no expert, but I was taught by a professional body man to mix the hardener "fully". In my experience, if the filler did not harden properly it was either my mistake in that I did not mix the hardener or I did not add enough.

#13 CASENUT OFFLINE  

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Posted September 21, 2012 - 10:59 PM

Really Bmerf...a lot of people skip this step and just mix it in...thinking' it'll be ok...

#14 Cvans ONLINE  

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Posted September 21, 2012 - 11:22 PM

Back when I was in high school and working in a body shop we used 36 grit sanding discs to prep the metal for filler. Never apply Bondo over any type of finish. The metal must be bare and clean.
  • johndeereelfman said thank you

#15 JDBrian OFFLINE  

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Posted September 22, 2012 - 06:09 AM

Thanks folks. I have learned a few things. I just find the whole process to be a bit wishy washy as described on the can. A golf ball sized blob of filler and 1 and 1/4 inch of hardener is what it says on the can. A golf ball as a unit of measure- LOL It sounds like something you have to get a feel for. The process of mixing the hardener, which is always separated, is a pain. I would gladly pay more for a product that was easier to use. I think I cut back too much on the hardener after last year when I was working in a hot garage and the filler was hardening before I could use it. As said, it's cheap, so just keep mixing more if it hardens too fast.
The 317 fender deck is a real mess. It's been bent and repaired and there are rust pock marks under the paint and bondo showing through where the paint is missing. It's also dinged in the back and will need to be straightened. This will be a real challenge and I thought I'd better get some info on how to do this correctly. As always, you folks have not disappointed. Lots of really great tips and info. here. I think I'm going to look into a better quality product for the fender deck. As for the MF8 parts, where I couldn't get the filler to firm up I removed it with acetone and started over. It's almost ready for paint but it's raining here and will be for a couple more days. I'm still hoping to get it painted before it gets too cold .




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