Cleaning removes the contamination; rinsing removes what the cleaning process leaves behind. Water quality is particularly important with ferrous metals, where chlorides and dissolved salts can promote flash rusting before the part is dry.
Two different questions get confused here: what water to make the wash bath from, and what water to rinse with. They have different answers, and the second one matters more than most people expect.
For the wash stage, ordinary potable mains water is normally the correct and economical choice. The chemistry is formulated to work in it, and detergents include sequestrants that handle typical hardness. Buying deionised water for a wash tank is usually money spent for no gain.
Two exceptions worth knowing:
Every dissolved solid in your rinse water stays on the part when the water evaporates. That is the entire mechanism behind water spotting, and it is why parts that were genuinely clean coming out of the wash can still fail a visual inspection after drying.
Deionised or reverse-osmosis water has had those dissolved solids removed, so it evaporates and leaves nothing. For anything with a cosmetic requirement, an optical surface, an electrical contact, or a cleanliness specification, a DI or RO final rinse is the difference between passing and arguing.
Here is the part most workshops never exploit. Water conductivity rises with dissolved ionic content, so measuring the conductivity of your rinse water tells you how much wash chemistry is still being carried out on the parts.
That gives you a genuine, recordable endpoint. Instead of "we rinse for two minutes because we always have", you can specify "rinse until the outgoing rinse conductivity is below X microsiemens per centimetre", prove it with a meter, and log it. On multi-stage machines the measurement can be built in and recorded automatically against each cycle.
It is the same argument as any other cleanliness verification method: a recorded measurement provides objective, traceable evidence of process performance.
Very pure water is a hungry solvent and will attack some materials, and it is corrosive to certain metals over long exposure. Use it for the rinse stage, not as a permanent standing bath, and specify tank materials accordingly.
A DI rinse tank that is never changed becomes a contaminated rinse tank. Its whole job is to be cleaner than the part. Monitor it — conductivity again — and change it on a rule, not a hunch.
A hot final rinse leaves the part with enough thermal mass to flash off residual water, which reduces spotting further and shortens the dry stage. This is why rinse aid and hot rinse usually get specified together.
Degassing a fresh bath · Rinse Aid · Temperature and substrate tolerance
We assess each application. Where required, we validate the process through a controlled cleaning trial on the machine and chemistry we would specify, with the cycle and the result reported against your standard — ask us to scope it.