Plugging a leaky radiator

My sooper-s3kr1t procedure for repairing a hairline crack in the radiator of an early 1990s Audi:

1. Notice the coolant warning light. Buy a one-liter bottle and top up.
2. Notice the coolant warning light again about a week later. Also notice the small puddle on the garage floor.
3. Panic.
4. Calm down a little. Breathe. Call a friend for advice.
5. Buy a stick of “chemical steel” (a two-part resin or putty that adheres to most metals and turns hard as steel in a matter of minutes) and a small jar or bottle of radiator sealant.
6. Audi radiators (at least from that era) don’t have filler caps. You top up the expansion tank instead. You should not pour radiator sealant in the expansion tank, though. This is very important. Coolant does not circulate through the expansion tank; it flows in when the pressure in the coolant system increases, and drains out when the pressure sinks. If you pour sealant into the expansion tank, it will sink to the bottom of the tank and stay there, and likely as not seal the outlet.
7. Think through the previous item, and decide not to use the sealant you bought.
8. Slap some chemical steel on the crack. This won’t work very well, because coolant is still seeping through the crack. Your fingers will smell very bad from kneading the putty, though.
9. Your radiator will still leak, but (at least to begin with) not quite so much.
10. Ignore the problem for as long as possible, until you’re spending almost as much on coolant as on gas.
11. You now have three choices: welding, chemical steel again, or the sealant.
12. Welding is the best option, but it’s complicated and expensive, not least because Audi radiators are made of aluminum, not steel.
13. Chemical steel didn’t work the first time; it’s not likely to work the second time either, but just in case, you can try again with a dry radiator. This means you need to drain at least enough coolant so the level in the radiator is lower than the crack. This is a good occasion to drain and flush the system and replace all the coolant.
14. The cooling system works as follows: coolant is pumped through the engine block and the sump by a coolant pump, which is driven by the timing belt. When the engine reaches its ideal working temperature, a valve opens and the coolant starts to circulate through the radiator as well. It enters the radiator at the top, makes its way through a long, winding pipe and exits from the bottom. The total capacity of the cooling system is about four liters.
15. I’m assuming you’ve removed the sound deadener under the engine. I leave mine off, it just gets in the way.
16. There is a drain tap in the bottom right corner of the radiator. Find some sort of container that will hold four liters and fit under the car. Place it under the tap and unscrew the plug (easily done by hand, especially if you remove the right fog light first). Nothing will happen.
17. Unscrew the filler cap on the expansion tank, letting air into the system. Coolant will now come gushing out of the drain tap, missing your container entirely.
18. Replace the filler cap, adjust the position of your container, and re-open the cap.
19. You have now drained the radiator, but there is still coolant in the engine. There is a drain plug somewhere, but it’s far easier to just disconnect the inlet or outlet hose for the oil cooling circuit.
20. Close everything, fill the system with water, and re-open. Repeat until the water comes out clear.
21. Reconnect all hoses and close the drain plug.
22. You can now try to slap some chemical steel on the crack again. It won’t help, but it’ll make you feel better for a day or two.
23. Give the chemical steel time to cure, then fill up with coolant.
24. The next day, go for a long drive. Notice the coolant warning light again. Swear profusely.
25. Decide to try the sealant instead.
26. In addition to the radiator sealant, you will need a meter or two of regular half-inch garden hose, a funnel with a stem that fits snugly inside the hose, and a three-liter watering can. You can do without the funnel, and you can replace the watering can with any sufficiently large recipient that is easy to pour from, but it’s easier with a watering can. In any case, make sure everything is clean.
27. Park the car outdoors and well away from any open door or window or ventilation inlet. Let the engine cool until you can touch the block and radiator without burning yourself. It helps if you let it stand with the hood open, since the hood is insulated.
28. Attach one end of the garden hose to the spout behind the drain plug (where the coolant comes out when you unscrew the plug). Stick the other end into the watering can.
29. Open the drain plug and the filler cap, and let the coolant run until the radiator is empty or the can is full, whichever comes first. You don’t have to drain the entire system, just the radiator.
30. Close the drain plug, but leave the filler cap open.
31. While the radiator is empty, you may want to remove the chemical steel. A single blow of a rubber mallet on a strategically applied chisel should do the job.
32. Unclamp the radiator inlet hose (in the top left corner of the radiator). Use a small socket wrench with an 8 mm socket rather than a screwdriver.
33. Remove the inlet hose. This isn’t easy, it’s a tight fit and there’s a rib behind the clamp, so make sure the clamp is wide open before you try.
34. Stick one end of the garden hose into the inlet. Wiggle the hose so the end points downward and get it as far into the radiator as you can (which probably won’t be more than around five centimeters, but try anyway).
35. Attach the funnel to the other end of the hose.
36. Radiator sealant comes in two types: liquid and solid (granules). If you bought the liquid type, shake the bottle thoroughly. If you bought granules, mix them with coolant (from the watering can) in a clean plastic bottle and shake until they are completely dissolved.
37. Hold the hose as high as you can and slowly pour the sealant into the funnel. Keep the hose as close to vertical as you can, and make sure it doesn’t loop.
38. Pour in some more coolant to flush any sealant that might be left in the hose into the radiator. You can rinse out the sealant bottle too if you want.
39. Reattach the inlet hose. Make sure that the clamp is in front of the rib and that everything is sealed tightly.
40. Pour the rest of the coolant you removed into the expansion tank—but don’t overfill it.
41. Start the engine and let it idle for ten to fifteen minutes. It needs to run long enough to reach working temperature so the thermostat will open and coolant will circulate through the radiator.
42. To begin with, the radiator will still leak. You will notice when the thermostat opens, because coolant will start to leak faster as pressure increases. After a while, it will slow down, and hopefully stop entirely. Use a clean rag or a piece of tissue paper to wipe away some of the leaked coolant. If you succeeded, the spot you wiped dry should stay dry.