The Stovebolt Site Forums The Shop Area Paint and Body Shop Need help with bondo

I am trying to fix some minor dents on the fenders of my truck and can't seem to get it right. Every time I add the "last" skim coat, I always sand it down too much and end up with low spots in the filler. I am using an eight inch long sanding block and everything looks fine until I shoot the primer, then the low spots show up. What is the trick to knowing when to stop sanding?

This is where it comes to kind of a "feel" for it. A few tips are the filler should feather off very gradually. If you see sharp or crisp edges on the filler to metal, you probably have sanded too much. In other words, if the plastic filler turns sort of transparent around the edges it is likely feathered off nice. If it is an abrupt end then the shiny metal starts, you probably have sanded the filler down below the metal height.

Take a smooth piece of metal with no dents. Apply a thin coat of plastic filler to it. Now sand it down, feeling it along the way till it is gone. This will teach you to recognize how to feel the filler. If you know some is till on that straight metal because you can see it, but you can't feel it, then you need to get more serious about feeling it.

Don't feel with the tips or your fingers or something. It is a common mistake for "newbies" to stick their hand straight out and rub the panel side to side in front of them. You can't feel a golf ball glued to the surface like that! You need to stand along side of the panel and make love to it. Rub the panel with a very light feel, from one end to the other of the repaired area. You need to start off metal, rub across the filler onto the other side on metal. Get a feeling of the flow of the panel. Go to another point that is identical like the other fender and feel the exact same place. See what it should feel like undamaged.

okay, since this is a bondo string, I have a question. Last summer I fixed the fenders on my '62. I wanted this a temporary fix, since I am searching for new fenders. I cut out the rusty spots with a cutting wheel. I used old license plates as backing and stuck them in there using JB weld. I then applied the bondo, and primed it after sanding it. This winter, the bondo has started to develope large cracks. Is this from shrinkage? did I not use the right mixture?

I dont know why more ppl dont use brazing to place your metal in holes. It is fairly easy can be done from the underside of any body piece, or actually cut to fit the hole.Brazing flows well and fills well, then all you have to use is spot putty to fill in the space and you dont have a part the is full of bondo. The answer to your question though is bondo does expand and contract with the elements, that is why yours cracked,the metal also has to clean sanded to the skin and I use paint prep cleaning agent made by centari to clean the area well before appling any plastics, which I thourghly despise doing anyway. hope this helps.

Alan

Jonathan, this could have been caused by a number of different reasons. Yes, polyester body filler is going to expand and contract at different rates than the metal it is on. But if used correctly, it will out last you and I, the truck will be rusting away in the wrecking yard and it will still be there! What you did was the just the wrong way to do it, and you knew it when you did it. You were planing on a temporary repair and you got it Believe me, I know, I have did similar things.

The object is to use as little as possible, this is true. I like to "metal finish" (repair the metal so no filler is used at all) whenever possible. But many times that is just not practical or even possible.

Next time you try to push the envelope of filler use try one of the water proof fiberglass or metal reinforced fillers. Evercoats "Kitty hair" or "Tiger hair" (you can fill holes 3" wide or more with Tiger hair) or "Metal to Metal." I have to say again, it is NOT the proper way to repair these holes (but you know that) but I have seen Kitty and Tiger Hair used to "repair" some pretty badly rusted things work dang good. Before I was repping paint, I would have never said this, but I have to say, I have seen it do some amazing things that lasted years.

Good luck.

Thanks MARTINSR,
I believe you hit the nail on the head. I am sanding away the feathered edge of the filler and getting it below the original metal panel. I guess I haven't developed the "feel" for it yet. If practice makes perfect, I should be really good at this by the time I finish.

What grit are you using for this final sanding? With too coarse a grit it will cut too much too fast.

Tony is right, if you have seen my "Bondo Basics" (which is posted here again) going up to 180 is the best way to go. This is for a number of reasons, one of them being Tonys point (one I never even thought of) the 180 cuts slower, allowing you more control.

yeah, I was going for a simple quick fix. Eventually I will get new fenders. I was just getting tired of looking at huge gaping holes in my truck. The reason I did this in the first place was that the guy who owned this truck before me had used bondo, and he did a worse job than I did. He stuffed old newspapers from the early '80's in there, and slapped the bondo OVER the rusty spots! he didnt even cut out the rust. When I got the truck, the rust had advanced and made the bondo ripple and get a festered look. Besides, the brackets holding the bottom of the fenders on had rotted away, and the bottom of the fenders stuck out half an inch from the truck. so I made temporary brackets out of some of that metal stripping with the holes in it (plumbers tape?) and just bolted it to the truck. right now it doesnt look half bad, as long as you dont look behind the fender!

I have been using 100 grit for the final sanding. While we are on the subject of sanding, here's another question. Is it better to dry sand or wet sand the final application of filler? I have been dry sanding but seems like I could get a smoother finished product if it were wet sanding.

DRY SAND. plastic filler is not water proof (well, a few are). You really don't need to get it any better than the 180. So instead of leaving it in 100, move up to 120 or 180.

While I agree with the dry sanding as the best way to finish bondo, I highly recomend using increasingly finer grits before applying any paint. I agree that 120-180 grit is good for the initial and rough work but to get a good finish finer paper should be used. Use 220 before priming and when using spot putty. You should finish off with 400-600 grit paper as a final prep before painting, and don't forget to use a tack cloth to remove the superfine dust from the area to be painted otherwise you'll get lumpy paint.

Good luck.

Problem is old Stovebolt trucks have rounded bodies & all the tools to do body work are flat.
Ya have to be somewhat of an artist to know how to work round body panels with a flat sander.

I know this is going to sound strange, but I look at a curved panel as sort of "bent flat panel". I block with flat tools and take an inch or so at a time. I have found that the rubber flexable blocks and things go in and out of the high and low spots, never really "flattening" them out.

MartinSr How do you feel about the paper wrapped around a paint stick.Can you get a a panel flat by doing it this way???

Sure Phat, but you would only use it in a very small area.

As a rule, you use as large a block as you can fit on the area being worked.

So the paint stick would be used in a very small area. It also works great for sanding out a run in the top coat for buffing.