My oil tank has a 'very slight oil seepage," the mechanic says. Why can’t this be fixed by soldering closed the leak? Or soldering a patch over it?
Is it your FUEL TANK or your OIL SUMP or OIL PAN on the bottom of the engine?
Or is it your tank for home heating oil?
Clearly, the word fixed is subjective 
Is there a crack in the oil pan? Or, is the oil pan gasket leaking?
If it’s a “very slight leak” you can let it be and check the oil level frequently.
Oil is flammable and you’re putting a very high heat near it, even though it isn’t an actual flame. The heat could cause the oil to catch fire and burn more than just a couple quarts of oil.
In addition to what the others have said, solder is not very strong mechanically. If the patch on this “oil tank” had to hold any weight or pressure, solder would not work. Also, solder does not adhere well to some metals.
Some other process (welding? brazing?) might be strong enough, but then you are back to the high(er) temperature issue.
j-b weld may work
Everyone has their “a expert old mechanic showed me this” story. A old guy I worked for would braze gas tanks (after emptying them) he would pump the exaust from a running car into the tank while brazing, I never saw one blow. He died of natural causes much latter.
I always wondered why it worked,displaced the vapors in the tank? dumb luck?
He displaced the oxygen out of the gas tank with carbon monoxide. And if there’s no oxygen in the tank, the fumes in the tank can’t be ignited.
Tester
The oil that has permeated the metal makes for a bad weld, so my welder buddies tell me. A temporary solution for oil and gas tanks is to rub bar soap over the leak area, it does not last forever but helped keep an old mercury stationwagon for a couple of years till I could trade it in! Beware of used cars would be my second advice. I was looking at used cars and saw a leaking transmission seal that the guy had tried to jb weld or bondo, I am not sure which, but it was not holding. Of course I passed on that car!
This summer I was doing something similar with a scrap deal I had. I was scrapping a couple of old tin/steel grain silos, you can’t just use a cutting torch because the mix of grain dust and air is explosive. We were putting pounds of dry ice into the silos. When the dry ice evaporated the heavy CO2 displaced the oxygen and let you do the cutting. Cut the bottom out and drop them like trees.
this is a touchy subject. most shops will NOT do a repair like this. this type of repair is only going to have the bottom of the oil pan fall off at a later date, when you least expect it.
i personally have used this type of epoxy mend to ‘skim coat’ the oil pan on MY auto. but this is just a stop gap, temporary measure, and I know full well the patch may fail at any time, under any circumstances. i don’t know of any real professional who would do this type of repair and charge for it.
i have never heard of ‘soldering’ a oil pan. i don’t think you could get the pan clean enough to get solder to ‘take’ to it so it would seal.
Siena–We are still waiting for clarification regarding exactly what you are referring to.
Fuel tank?
Oil Pan?
Home heating oil tank?
A Russian T-55 tank?
The first gas station I worked at used to weld/braze gas tanks all the time. They would first flush them with water, and then, as oldschool noted, would pump exhaust from a running car through the tank to purge the oxygen. Even with those precautions, there was often a loud “whooosh” when the torch flame hit the tank. It was loud enough to make me stand back.
Another trick I’ve successfully used, depending on the crack, is to drill a hole in the tank or pan, then self-thread a sheet metal screw to it with a large firm rubber washer on the end. The screw would hold the washer firmly in place and stop the leak. This wouldn’t work with rotted out areas or long cracks.
The only thing missing from this thread is a Original Poster…We still don’t know what kind of oil tank he is trying to solder…
Oil tank? I’m guessing the OP has a motorcycle with dry sump lubrication where the oil is kept in a separate tank outside the engine. Harleys, old British bikes, the original Honda 750 fours were dry sump.
If the crack is due to vibration, that crack will just continue to spread unless you arrest it by drilling a tiny hole at each end of the crack to get rid of the stress risers. Then you can repair it with solder. Good luck!
Another example of an OP asking a general question and never following up. Rocketman
That Hasn’t Stopped This Crew From Generating 18+ Answers, Though.
Who needs details?
What kind of car has an oil tank?..or is this a motorcycle you are asking about?
If you are referring to your oil pan, it is probably the gasket seal that is leaking, and gasket seals are not supposed to be welded or soldered. Leaking gasket seals are supposed to be replaced with new gasket seals.
It might help if we knew the year, make, model, and odometer reading.