Radweld is a last resort. It will settle in the water passages in the block and radiator and heater matrix and clog them up a little.
The system should always be run pressurised. This has several benefits to you notably that the system should stay sealed better for longer, unless there's a leak somewhere, in which case NOT pressurised is better.
If you are running with a loose radiator cap, expect to loose coolant, and what you lose will be the anti-freeze first, especially if it is the cheaper methanol-based stuff. Methanol boils at a mere 65 degrees so as your engine is set to run at a steady 85 degrees, the methanol soon vanishes as a gas out of the expansion tank and the loose cap. If all is well, the expansion tank should be nearly cool enough most of the time to hold the methanol vapour in if the cap is good and tight, and holds the pressure.
So, as you will also lose some water by vaporisation (even if it doesn't boil) the loose cap can make it appear there's a leak when there isn't.
CharlesY