Hello,

I have a 1973 F250 that used to have a gas tank behind the bench seat. Someone removed the tank at some point, then decided to fix it with bondo instead of welding in a patch. This is what I'm working with:








In general terms my plan is to remove the bondo plug, sand a small area surrounding it back to bare metal, weld in a circular patch, grind it flat, smooth out with body filler, and paint. However, I could use some advice on the specifics involved. The current paint job looks like a single stage rattle can, so I'm not too concerned about a perfect outcome.

1. Am I going about this the right way?

2. Would you use sandpaper to remove the bondo around the area? Or a die grinder? What would be the best tool for the job?

3. Once the patch is welded in and ground flat, would you prime the bare metal, then use body filler? I don't want it to rust since I live in New England...

I want to go in with a solid game plan before I start hacking at it. I'll have to learn as I go, but any tips and advice would be much appreciated. I'm especially interested in what NOT to do as well. Thanks!

 

I'm not a body guy but learning as I go. Your plan sounds good to me. I used a paint stripping disk on a mini grinder and the same type of material on the drill. Looks like black string that is glued together. Works great. I used an epoxy primer (2K) then bondo, then paint. Make sue to prime the back side of the metal as well. Could use weldable primer for that since it will burn when welding. Go slow with the welding. Tack, move 1 inch, tack, move 1 inch, tack. All the way around. Hammer and dolly the welds, then grind down the welds, then repeat, going half way between the first welds. Weld grind repeat. Then you could start at the first weld dot and put a second one right next to the first one or go half way between. Does that make sense?

 

Thanks for the advice! So when you say to hammer and dolly the welds, is that to knock slag off or to depress them? And you would grind the tack welds before adding new ones?

 

I'm assuming you are using a mig welder. There is no slag when you use one. There is always some shrinking happening when the weld cools. It will pull the metal into itself. The idea of hammer and dolly is to squish the tack a bit to expand it again. Kind of like hitting your finger with a hammer, it gets bigger, and squishes it. lol. Yes grind off the weld before the next round of tacks. That makes sure you don't hit the older tacks when you hammer and dolly the new welds. There is a guy on this web site and forum named MP&C. Not sure if he owns a body shop or restoration business/shop but he is really helpful with this stuff. Most of what I am saying started with watching you tube videos, esp, welding tips and tricks. Guys name is Jody. And MP&C refining the way I do things. I'm still learning but getting better then when I started. The other thing I do/did do is to practice on some scrap metal. Another thing, always butt weld your patches in, never overlap them. It will expand and contract differently in the sun and you will notice it. Also don't use square patches, round off the corners say 1" radius at least. That will help with heat transfer.
Here is a picture in progress of my fender that I widened.


Picture of my cowl vent patch before I started welding it in.

 

Thanks for the great explanation! It definitely helps to know why you're doing something. Also, cool pictures. I'm always impressed by what people on here are working on. I'll be posting here as I go through this process (hard to find time with work).

 

Marten, thanks for the kind words.. Glad my ramblings helped out.. FYI, small restoration shop.

Richard, on this thread starting at about post 36 there are some pretty detailed explanations that may help you out... Although the entire thread has some good info as well.


https://www.ford-trucks.com/forums/1...-panels-3.html


See if that clears up some things.. On your primer vs filler first, I would suggest a good epoxy primer like SPI first, then filler over top of that. If you use filler direct to metal, the chemical reaction and heat byproduct of it tend to cause condensation on the steel panel. Grind that filler off later and you'll see rust has formed on the surface of the steel from said condensation, compromising the adhesion. So epoxy first then filler over epoxy. On a weld through vs. epoxy, either is going to burn at the weld. A restoration publication I read a few years back did a test study on long term protection offered by epoxy vs. weld through and found that epoxy was the better choice. IMO you won't find anything in a spray can that will out perform the durability and protection against rust creep as you have in using an epoxy primer. I pre-coat anything that will be difficult to access later, but you'd be amazed at how disposable acid brushes can be bent to paint in nooks and crannies, and I've even made a 4' long handle on a 1-1/2" wide paint roller to epoxy the insides of rocker panels after installation. They had already been coated, but the additional coats help to seal off the seams. So if your next "excuse" is that you don't have paint guns, air compressor, etc, epoxy primer doesn't care. Brush it on, roll it on, spray it on. You are likely going to block it out regardless, so who cares if there may be a couple brush strokes or dust nibs from a paint roller. Having a good quality primer as your substrate is the key more than anything else.

 

Great information! I see Marten's praise was well earned. I haven't tackled this yet since I found out I need to work this weekend. But I'll be posting when I start. No doubt I'll run into issues I need advice on.

 

I will echo what MP&C says about primer then bondo. The guy that I buy my body work stuff from uses a fixed up Ranger p/u. It was a roll over accident. So both sides were damaged the same. He did one side in primer then bondo and the other side bondo then primer and that is how he drives around. The primer first side is still looking good. The bondo side first is all chipping off.

 

Just an FYI. Not starting the great debate.

Good link for the old debate of filler first vs primer first. My experience with this, is filler first, then epoxy prime. Use a good filler. 3m, rage. Etc. Proper prep work is the key to all of it.

Epoxy or No Epoxy Under Filler - Autobodystore

Lots of other good links there for the OP.