Which shops clean Thai white formal uniforms, and how is it done?

It is perspiration that has oxidised. It sits in the collar and the inside of the cuffs, and ordinary washing tends to set it rather than lift it — which is why a uniform can come back from a general laundry with a clean body and an unchanged collar. It needs a targeted whitening step applied by hand to those areas specifically.

Detergent residue left in the fibre continues to react, the plastic cover it is hung in traps moisture and off-gasses, and Bangkok humidity accelerates both. This yellowing is spread evenly, so it has to be corrected across the whole garment. Correcting only the collar produces a white collar on a cream jacket, which looks worse than leaving it.

No — this is the single most damaging thing you can do. Chlorine bleach reacts with the poly-cotton these uniforms are usually made from and turns it permanently yellow, which is precisely the problem you were trying to solve. It also attacks any gold or silver braid. If a shop offers to bleach a white dress uniform, that is a reason to use a different shop.

They should be photographed in position, removed where they are designed to be removable, and shielded where they are not, then put back exactly as they were. Metal buttons, shoulder boards and rank markings tarnish, snag or distort under a normal cycle and a normal press. You do not need to remove them yourself, and doing it in a hurry is the step most likely to tear the fabric.

Usually several shades, and sometimes fully — but a uniform that has yellowed evenly over many years may not return to the white it left the tailor as. Anyone who promises otherwise before seeing it is guessing. Heat-glazing from a domestic iron is separate and is not recoverable at all: a shiny patch on white fabric is crushed fibre, not soil.