It drops from max to min over a period of six months...? I wouldn't consider this to be a problem. Surely it can only be a matter of half a pint or so, and I would think a passat has quite a large capacity coolant system anyway. I think that putting radweld in will do more harm than good.
I've used radweld for years and had no problems. However, you may not have a problem. The coolant on my wife's Polo needed topping up from Min to Max every few months. I could find no leak anywhere, so left it to see what it would do. The level did not drop below the Min mark and it has stayed on Min ever since. The system seems to have expelled the excess through the pressure release valve and it was only when I examined the valve closely did I see the tell-tale pink crystals of antifreeze around it.
Some points relate to a leak.point was made over six month period.unless any one expects levels to stay the same in this time.would think they are wrong.too many variants to say it is correct.My previous post stands.
I've tried rad weld once, with no joy. So when i had a leak problem on my Golf 2 GTi, I tried a Sodium Silicate (water glass) based product. It worked a treat and the leaks stopped. You have to flush the coolant, fill the system up again, with the stuff added, run it for half an hour, then drain the excess and leave to stand for 4-12hours to harden. Then you fill up with water and anti-freeze and away you go.
However, I started loosing water again recently (after 12months) which turned out to be from the heater matrix as mentioned in a different thread. I had already bought another bottle of the stuff (approx £10), trying not to have to change the radiator.. but then realised the real cause.
I now have the a spare bottle of the stuff. I will check its name and put it on here tomorrow.
See if you can get the cooling system pressure tested. Radweld and similar products are all good emergency 'get you home' type treatments but they don't cure the cause of the leak - only the symptom. If you intend keeping the car for a length of time and want the normally reliable Passat to remain reliable, then find the cause as it will eventually get worse. A pressure test will normally produce leakage from somewhere and if not then suspect the coolant expansion tank cap. These do wear out due to constant cycling of heat, cold and pressure. Waterpumps can leak intermittently under the timing belt cover where engine heat evaporates any traces.