oxfordbp

I thought you shouldn't use filler over anything BUT bare metal to get the best bond?

JMD

For older filler it was always recommended to got to bare metal. Even so, I dont think that this had much to do with the filler itself, more so to the surface it was put on. A chain is only as strong as it's weakest link. If the paint or primer on a car is prone to lifting, so to will be the filler put on top of it.

Filler was introduced in the era of lacquer primers and when it was not uncommon for topcoats to be unactivated. In my opinion, so long as the base has excellent adhesion, the coats of filler are thin, and the base has at least an 80 grit scratch, the filler will stay without problems, whether put over an epoxy or a good 2K primer.

Lots of folks will disagree in theory with what I said, but I bet these same folks "spot" a little filler over 2K in the blocking process....

It is my understanding that Rage Gold is formulated to go over Epoxy primer, and the Z-Grip I mentioned is formulated to go over bare metal. Frankly, I don't think it matters much.

In a practical sense, if you go over epoxy, and don't get the filler down in time, you need to have a good 80 scratch for adhesion, which often will get you back to at least a little bare metal....

"Bondo failure" is rarely a result of poor adhesion, it is usually a result of poor prep and/or too much being used, and/or rust working through the back side from holes covered up or "filled" with filler.

IMO, either filler over epoxy or over bare metal, properly used and applied doesn't make any damn difference...

NoReins

If you use the epoxy from Southern Polyurethanes, you can spray the metal and then apply the filler the next day. You can wait up to 7 days without having to scuff up the epoxy for more filler. After 7 days you simply just need to scuff the epoxy with 180 grit sand paper.

The thing about epoxy is it is moisture resistant, unlike your urethane primer (2k) and also filler. We pulled our car on an open trailer with epoxy on it for 20 hours, it rained heavy for 13 hours straight. The water just beaded off. Both 2k and filler absorb moisture. It just makes sense use epoxy as the first coat over your bare metal as it will seal off moisture from the start. After you spray the epoxy, and then apply your filler; seal it off again the next day with another coat of epoxy.

mr_velocity

I'm still not a believer in not sanding epoxy primer before applying filler, even if its within the window. Epoxy is just that, epoxy. All the fillers I've seen are polyester based, poly will not chemically bond to epoxy, it needs something to grip that being the scratches from sanding (mechanical bond). I'm not sure why epoxy fillers aren't more prevalent today but cheap poly is still mainstream. All boat repairs I've done I've mixed up epoxy fillers.

Just my 02, could be totally wrong just going by what boat builders do that require high strength bonds between laminates.

I used Rage Gold over sanded zinc chromate epoxy.

JMD

I AM NOT saying that a coat of epoxy primer over bare metal is not a good idea, because it is, I am just saying that I think filler will work just as well over bare metal as over epoxy....

rmodel65

it will...but putting down that layer of epoxy seals the metal from the elements whereas wiping filler letting it sit open unprimed etc will absorb moisture which will make it to the sheet metal (unless youre using something like kitty or tiger hair that doesnt)

JMD

But doesn't this make the problem leaving unprotected filler exposed to the elements?

I mean say a person uses epoxy primer, applies their filler and then leaves the car out in the elements for a few weeks. The filler will absorb some moisture, and the moisture might make its way to the epoxy primer, thus stopping the water from oxidizing the metal. This is good, the primer has done it's job.

But still, the filler now has water trapped in it, now if the filler is finished and top coated, the water is going to want OUT, and it will lift the finish.

In my mind, this makes keeping moisture from the surface of the filler of much more importance than sealing the filler from the metal. IMO, if a car is going to be left in the weather during the bodywork process, I am thinking that it would be more important to get the epoxy OVER the filler than UNDER the filler, kind of like stopping the train before it gets to the station......

Anyone follow me here...

Everyone agrees that filler absorbs water. does this mean that if someone gets water on, or wet sands exposed filler that it all has to come out? I don't think so. The problem with porosity comes mainly when unprotected filler is left in the elements for extended periods of time, (like days or longer), where repeated heat cycles and a regularly appearing (i.e. rain or dew, etc) film of water cause the filler to wick moisture. So in other word, the guy who does the bodywork on his 62 Chevy in the driveway over a period spanning several months to several years is the one who is going to have issues with water leaching through the filler and rusting the car body. The person who keeps their car out of the elements, not so much.

rmodel65

epoxy top an bottom best of both worlds

JMD

Here is my short take on the original question, Don't use bondo, it is too damn hard to sand and shape, most filler priced at $15 a gallon and up (at my paint shop anyway) will provide satisfactory results.

As to whether "bare metal" or over epoxy, either will work fine so long as you apply it on a surface with enough "tooth" to hold it, but over epoxy being marginally better if you keep the car out of the elements in the process.

Some years back I remember watching an episode of "Monster House", one of those shows where a crew would renovate a house in a week. On this particular episode they were building a new staircase partially cantilevered over the first floor.

The building code required that all welds had to be made with SEVEN passes. The welder on the crew had to spend the entire week on this relatively small project.

The thing that struck me was "seven passes? you have to be ****ting me". Yea this was in earthquake country, southern California... but seven passes? Making a weld joint 10 times stronger than the parent metal for a residential staircase is just dumb and a waste of resources.

Sometimes it is just too easy to get caught up in the process.

There is a point where and abundance of caution, and "doing things by the book" just fails to make sense.