How to patch holes and other bodywork that's long-lasting?

[ELS]

I usually see people tack weld sheets in, most commonly on top of an edge of the previous metal, but more professional shops put them edge-to edge.
in non-exterior patches some put seam sealant in the seams.
in exterior patches most put bondo on bare metal and then paint it...
Hammerite paint is supposed to be rust-converting but in my experience it still rusts, only thing that I've found that actually stops rust for a good while is rust-converter, I found this stuff called "Buvanol" and it works great, once I had a bit left over and I just brushed it onto some absolutely rotten spot on a fender, it turned from rust color to dark-blue, and with no paint, and being left outside, it didn't start rusting for 2 years.
But I digress.

I've never seen a patchwork technique that doesn't start rusting almost right away.

If I need to bondo the welds or whatever, do I first put on a primer, maybe a brush-on thick primer that I sand flat afterwards to make sure there's paint around the welds and in the seam?
If it's in for example a frame rail, how do I make sure the inside doesn't rust, when the inside cannot be painted? Do I paint before welding? or something else?

One technique I've found that does delay rust a good amount is coating the metal with bitumen, it has to be liquid enough to be tacky when cool, the solid plates that are often used from the factory for dampening sound just trap moisture and actually cause rust. But that's only viable for the interior or other places which aren't visible.
What can you suggest?

[mt-engines]

Bondo is hygroscopic. And attracts moisture, thats why often you will see rust behind a patch of bondo. Some guys prefer to use a panelbond adhesive, then use a sealer, followed by filler and or a 2 k surfacer

[Tom68]

Where are you working ELS, humidity is a big factor in all this.

We'd have half a dozen of these on the go at any one ime back in the day, when the humidity came the topside of all the horizontal surfaces had to be kept oiled otherwise they'd rust overnight, when the airs dry you won't see rust.

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[ELS]

Where are you working ELS, humidity is a big factor in all this.

We'd have half a dozen of these on the go at any one ime back in the day, when the humidity came the topside of all the horizontal surfaces had to be kept oiled otherwise they'd rust overnight, when the airs dry you won't see rust.


20230412_185552.jpg

I'm from Latvia, it's northen europe, near the sea, humidity is a big problem, in fall it's just a constant 100%
I'm not so worried about the added weight of some rust protective coatings, but 200 pounds of paint isn't appealing...

[F-BIRD'88]

If the patch is lap welded any water leak from the back side will get between the metal patch and bondo. Rust will form and the bondo will bubble.
Double trouble if winter driven with salted roads.

Greasing the back side/ inner panel of all patches helps keep the water out.

Any car that is/ was rusted or driven in salted environment will see further acellerated corrosion from the back side / inner panels.
Rust Check helps a lot. Must be applyed annually, repeated at least every year.
www.rustcheck.com

[F-BIRD'88]

Often on lapped welded patches that patch edge will become visible in the painted finish over time.
You can see the patch slightly in the finish.
The bondo shrinks.

[Sst3193]

I’m not a body person and honestly don’t care for that part of the auto hobby. Seen Foose’s gang and others, cut a patch to exactly fit a hole and weld the metal all around then, grind it to a finished surface. Do those repairs show, after the paint finish is cured?

[modok]

Repaired panels should be worked so you don't need more than 2mm of filler. I don't do any lap joints.
Marine fairing compound epoxy can be used instead of polyester filler.

One thing I have found is if it rusts out in an area once, it will do so again.
So those areas need a different strategy.
I may just start using stainless steel, or sacrificial anodes, or aluminized sheetmetal.

[HemiJoel]

If you have access to a facility that does chemical stripping and E-coating, you weld in a patch with butt welds, metal finish it to make Bondo use minimal, have the panel chemically stripped, then E-coated. Then thin Bondo if needed over the E-coat, then the usual prime, sand, paint routine. With the E-coat, the metal is all sealed, even seam and folds.

[HemiJoel]

For rustproofing, I dump 2 cans of STP into a gallon of WD-40 and mix. Then use a paint spray gun to fog all the inner panels, underside, for folds, etc. The WD evaporates off, and leaves a very thin film of STP. It really works, it lasts for years, and cleans off easily with solvent if need be.

[EXCELLENCE AUTO]

Often on lapped welded patches that patch edge will become visible in the painted finish over time.
You can see the patch slightly in the finish.
The bondo shrinks.

I learned in the 80s to put down a coat of KittyHair, or another fiberglass filler directly over welds on repair panels.
This included when we would shave door handles, or do other "custom" metal work. Then followed by traditional methods of filler will eliminate seeing the patch edge over time.
Of course, seal the back side of the repair and you should have a quality repair that lasts for well over 30 years. Depending on how good the repair really is.

[ELS]

Well it seems the most popular options are boiled linseed oil (BLO), was used to coat frame rails in aircraft to keep them from rusting due to condensation caused by altitude change.
wool oil, because it creeps into spot welded joints.
pitch/tar thinned in diesel or gasoline or turpentine, etc.

It seems the trick is to have something that creeps in every joint, but that doesn't wash out with water.
Then for the undercarriage have something thick and soft that can handle some gravel bouncing off it.

For interior that makes BLO pretty much the best option it seems, since BLO dries (BLO is the base for oil paints), and it's quite thin.
But since it dries it also could crack with vibrations, than perhaps mixing BLO with some wool oil would keep it slightly soft, although idk the chemistry; that might break the reaction that dries that oil.
Also if time is not of the essence, regular linseed oil takes like 10x longer to dry (instead of a week, months!), might be easier to get.
And for the undercarriage pitch/tar seems a good candidate, although I've found that it accumulates dust and turns dry quite quickly.
I've also read about using coal tar, but I have no idea where one buys such a thing.
Maybe coat everything in BLO, then coat pitch on top of that.

Oh and for the pitch, you could use gasoline or something like that as a temporary thinner so you can get it into weld seams, but also use something like diesel to thin it in the long term, otherwise it becomes too dry and moisture gets behind it and rusts the steel even more.

For the interior maybe coat some type of wax on top of the BLO, because wool oil would probably smell like sheep, while beeswax would be much more pleasant.
And as we know from GM wax frames, it lasts for a good while, considering it's just wax on steel.

Also there is a thing as moisture drying paint, used on bridges apparently. quite uncommon to come by but it sounds like it's just what you would want for a primer on the undercarriage.

And for paints, high zinc and/or lead content improved rust proofing, although lead paint tends to crack so that pretty much leaves zinc rich paint on the table. and ofc lead paint isn't on store shelves anymore.
Often used to paint ships and stuff like that with zinc rich paint, usually very pricey.


So get yourself a can of zinc rich leaded bitumen coal tar wool oil boiled linseed oil moisture drying ship bridge paint, and it should last. :D

[ELS]

And about the bondo thing, idk yet what's the best way to go about it... it's almost like all the bondo's that there are, aren't good enough.
You'd probably want something that "sweats" a thin oil when you apply it.
Thick oil paints for paintings come to mind... they're just BLO mixed with a pigment, but they sort of "sweat" the oil. So maybe take some of that, mix it with that sodium sulfate or whatever the sanding filler grit is, and apply that.
You can still get leaded painting paint in places, but lead tends to crack after some time.

There is the welding primer, which is just primer with a lot of zinc in it (zinc is a type of healing paint, so when you damage zinc paint by welding near it, it melts and fills in the gaps, healing itself)
But spraying it is marginal ofc, you could try to cut open the zinc primer paint can, and apply it with a brush. It's tricky getting it open without getting paint in your face, especially with a full can of paint. But the idea is you punch a small hole at the top of the can, since the paint will be at the bottom.
But don't punch it near the middle because you could instead punch a hole in the tube which would obviously spray paint everywhere.
So hold the can in a vice securely, put on some safety glasses ofc, and punch a pin hole around the rim on the top of the can.
That works much better with empty cans, have done it many times with empty ones just to get every last bit of paint out.

The paint inside is extremely thin, so take a brush and just flood the weld seam with it, then store the paint in a jar or something.
That should probably be the best priming method before bondo for weld seams.


Also, ofc just before you apply the bondo you'd want to clean it toughly from any grease, white spirit seems to be the most common for this but any thinner should work.