Ideally all plastic, rubber, or other non-metal parts should be removed before soldering although often this is not possible. A faucet or hose bibb or in line valve that is soldered (sweated) on should be in the open position.
The advent of lead free solder has increased the chance of damage to valves and other fittings during soldering. Electronic solder, which has a lower melting point, is safer for the valves and fittings although that joint would have a tendency to come apart at the moment another nearby joint was soldered using regular plumbing solder.
When cleaning anything including baby diapers, it is better to sweep off or scrape off as much solid and semisolid material as you can first. (1) Less cleaning fluid or detergent needs to be used, (2) Less of a mushy mess which in turn means that bleaches or disinfectants will be more effective.
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