[NudeAutoMall]

Having a problem with bondo cracking. (Actualy I am using Evercoat) We have had a long heat this year, three months in the tripple digits, and the filler I put in this spring is full of hareline cracks; cracks like dry dirt cracks, no relation to the body work beneath.

I removed bad filler like this origionaly, cracked not in relation to the dammage beneath. The old metal work was bad, they had just bondowed over broken metal. I pounded everything out, weleded everything together, then ground everything smooth and shiny.

I first ran into this when selling a rare old car for an estate, bondo cracking and pealing unrelated to the repares below. There is rarly any rust underneath as things just don't rust as easily in the desert.

My question is: is there anything to prevent this shy of fireing up the toarch and leading her, or completely perfecting the metal to need no filler.

 

[302 Z28]

Almost all filler cracking issues can be traced to one or two causes. One is it was applied too thick. Two is it was mixed too hot meaning too much catalyst was added and it over reacted. When the outside temp is in the triple digits it is prudent to reduce the amount of catalyst you would normally use. It is far better to extend the cure time than to rush it with too much catalyst.

Vince

 

[MARTINSR]

Personally I have never seen filler fail without metal moving somehow under it. OR applied REAL thick like an inch.

I would bet money that the metal IS moving somehow.

Brian

 

[Lhorn]

That would be my guess as well. Perhaps the metal is flexing a bit. Could you post a picture of the area.

 

[NudeAutoMall]

The current one is primer black, not photographing at all.
here is a pick of the first time I ran into this, although not all is showing in the photograph, especially the way the cracks peal up at the edges. This was a Kubel restored for a Museum many years ago.

I recently ran into this in an Cady that had an older restoration in Vegas

A beat up old Chevy Pickup had the same problem.

And the current Jag.

Movement in metal I would agree to be part of the problem, with not uncommon 50° 60° F changes from day to night, I'm sure there is expansion and contraction in the metal; that along with the extreme dryness I'm sure is the cause. All of these cars... were stored outside when the damage occurred.

Here in the desert you don't put a lot of hardener in if you want to get the filler off your board.

I have been throwing mud for about 40 years, and I typically use about 10 gallons a month. My Baby Brother goes through about a pallet or two a month. Not that I know it all, just that I have some experience.

I agree thick bondo has a tendency to crack, but not necessarily. Elbondomino I built when I was but a teenager, 5 gallons in her rear quarters. I ran into Elbondomino at a Cruse in back east some 30 years later, she had not a crack in her. Likewise if thick bondo always cracked Pattern Makers would be in a lot of trouble as originals are commonly carved from bondo. The second picture I have attached is a faucet handle I carved from bondo a little over ten years ago, is over an inch thick, and shows no signs of cracking.

Thick bondo cracks because of moisture. Like a sponge bondo is capable of absorbing large amounts of water, however bondo will swell extremely. Just drop a small chunk of bondo into a glass of water and let it sit for a few days.

I do thank you all for your input, but I am looking for maybe a brand, or additive, or trick that is more Desert friendly?

 

[Old Fool]

Just a wild guess from way out in left field, are you using a metal prep before application of any primer or bondo?

 

[NudeAutoMall]

No, but I did use rattle can primer to prep her for a shoot, have since sanded that junk off and replaced it with a good primer. Forgot about that till you mentioned this. That would be nice if that is all this was. And, she has cracked long after the good primer. I know the pigment is nill in that junk, but is the thinner that strong?

 

[BarryK]

What Old fool said would have been my first guess as an acid film will or can cause a polyester (body-filler) to cure on the outside and stay soft on the part next to the metal. Just as an acid film will do the same to a polyol (primer) or epoxy resins.

Now over the years I have seen fillers crack that were applied to rotisserie type vehicles and then sat in a corner of the shop for six months.
I looked at a 57 T-Bird that had been setting for six months this way in a shop and very thin coats of filler but all had hairline (fine) cracks, scratched my head but then the filler regional manager came out and said unprimed filler can do that. Kinda makes sense to me as the filler must retain some solvents or like a plastic dash can crack.

You may not have done anything wrong here and the filler may not be at fault either if the above scenario fits your situation, I would call the filler company as they are the only ones that can give you the correct information as to what happened and why.

 

[NudeAutoMall]

Thanks BarryK, Great food for thought.
3 months with 100�+ temperatures sitting out in the sun, but always under primer.

Moving to a ten car shop and away from this two and a half car joke. I'll still be working on some cars outside but nothing like I have been.

The Teck Reps back east are great, but out here we have more Sales Reps. For instance, last month I asked for 17.5" 80 grit file paper and got a roll of 16" masking paper. You can almost see where the loss in the information stream occurred, but frustrating!

 

[MARTINSR]

Just call the tech line from the number on the can!

Brian