Bondo issues...

[sublimer]

When I bought the truck, a previous owner did some bondo work a few places around the body and then did an okay paint job on top of that...now the paint where the bondo was is chipping off and some places (like the hood) have cracks in the bondo.

Is this because the bondo was used wrong, or does this just happen with time? Repair suggestions?

[fordman]

it may be that he didnt properly prepare the bondo for painting. thin bondo is what is used for areas like the hood and roof. he may have also used the wrong bondo.

[sublimer]

I've never worked with it before, but I take your response as meaning that this isn't a normal thing for bondo.

[Dan]

This is why it's best never to buy a vehicle that has had previous bodywork (although it's hard to avoid). Bondo is almost always used improperly, put on too thick and the surface area not prepped correctly.

It can also be mixed wrong (2 part--putty and a hardening agent) so it can under or over cure. Bottom line though, even when put on properly, it seems like sooner or later it will crack or bubble. And typically there is even worse rust or damage underneath because moisture has gotten in under the bondo and been trapped there, damaging the repaired area even more.

If the original rust or repair wasn't done properly, the bondo over it will make it MANY times worse, often not worth saving the 2nd time around.

Then to add insult to injury, many people decide to remove the bondo and attack it with a grinder or D.A. too aggressively and warp up the base metal. Bondo can make a car look great for sure, but it very often is one of the worst things that can happen to a car if used improperly.

[70 ranger]

I would look into the paint and primer first. you said it has an O.K. paint job?? is it a quality paint? The reason I ask is because plastic fillers by any name absorb water. They need a quality two part primer followed by a single stage two part paint or a base clear system. Paint will absorb water eventually in some conditions as well.( Ever seen a car with a tarp on it for a winter or two develop moisture bubbles? Or remember those magnetic signs everyone had years ago on their doors? when they removed them because the lease was up they got quite a shock!) A poor quality paint will allow the water to penetrate into the filler. When you remove it you will find rust on the surface of the metal underneath the filler. Unless there is a hole in the metal under the filler that is the only way for the rust to get there. When you remove the paint to repair these spots I would check for paint that does not rub of with laquer thinner. Primer that does not rub off with thinner. (If it is soluble in thinners it means no hardner)and I would check for etching primers or epoxy primers on the metal surface. If these steps werent taken you might want to strip the whole panel and start over with quality stuff.