bondo issues

[d240man]

:stupid: I have a small dent in my 73 240 hood and everytime i bondo it , it slowly come out. I took the hood down to bare metal used a metal liquid cleaner and then with the help of my 4.5 grinder, roughed up the metal. Applied the bondo, one time in several layers and the next time in one application. After about a month it starts to crack, then you can peel it right out. What am I doing wrong? Outside of using bondo in the first place.

[kmack]

First off, never apply filler directly on top of bare metal. Metal can actually absord moisture which can cause the filler to "let go".

When going to bare metal, use a good etching primer and sealer first. Then apply the filler. I use a product called "Ospho" which is a mild phosphoric acid to rid the metal of surface rust (which btw, absolutely has to dry for 24 hrs). Then I coat with a 2-part etching primer/sealer. Then the filler goes on top.

The optimum thickness for fillers is 1/8" or less. Anything thicker than that and 1) you haven't prepared the metal correctly (as in you haven't straightened it enough), and 2) you run the risk of the filler expanding or contracting different than the metal, which can cause it to let go.

[carguyinok]

bondo :disappoin

Why God why did you let this stuff come out from the deepest parts of Hell?

If you cant tell I am NO fan of this stuff. It can be good IN THE HANDS OF A PRO BODY PERSON. But there is no way stores should sell this stuff to just anyone. O.K. Sorry.

On your hood. Where?

From the sounds of it the bondo is in A spot on the hood where there is A big change in temp. With this temp change (bondo) will expand and contract with temp changes. With that there is no way the bondo can keep hold of the metal.

Why not tap out the dent? There are other things out there where you can also (suck out) the dent almost like A plunger type deal. Another one is A glue on fitting that will let you pull the dent out. Better yet have A body shop fix it. A good shop wont want A ton of money for A small fix. YOUR Z IS WORTH IT. Rite?

[kmack]

Also, a "small dent" can still mean a fairly larger area of filler will be used.

I had a dent in my hood that was about 3" in diameter. after all the filler work, I had a spot that was almost 12" in diameter!

[EScanlon]

The jury will always be out on bondo.

Where years ago, metal craftsmen would carefully and laboriously restore the sheet metal of a car using pick hammers, slap files, melted lead and very careful cooper like techniques, that has been simplified to grind, slap a coat of hardening polyesther resins and shave and smooth to fit.

Sadly, very few of us can afford the former treatment, and must instead rely (at least on our newer cars) on simple replacement, whereas those vehicles for which off the shelf replacement parts are no longer available, we must therefore resort to this "lost" art.

The main reason your "patch" isn't sticking may be because of the difference in expansion coefficients, or it may be as simple as that your hood is "washboarding".

That's a term for what happens when the metal panel is pushed against the normal contour and it "pops" or / "boing" into an inverted, or deformed contour. This is common on large flat panels. Roof, Hood, Hatch, Doors, Side Panels etc.

The patch will not be able to stretch / shrink when this happens. It eventually gets cracked / loosened enough that it suddenly "pops" off.

You need to bump back that "dent" while shrinking the metal. This is where body men use that "meat tenderizer" hammer / dolly. Depending on what needs to be done, sometimes doing a quench shrink is what is required. Until you've stiffened the metal again you are going to have a problem.

There is also another technique whereby you counter strike the dent in order to eliminate the washboarding, although you don't shrink the metal and you still have a sizeable thickness of bondo. Not the best of repairs, but it is an Earl Shive Technique.

2¢