I recently acquired a steel fork, but the paint is very damaged and I would like to polish it and have it silver colour.
What is a good system to have it nice as the one they sell, which look so bright?
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Shiny metal parts on a bike are either chromed steel, stainless steel, aluminum, or titanium.
Bare steel can be polished to a shine, but the shine will only last a few hours (if that). Steel naturally reacts with oxygen in the air and turns to rust, so any steel that is not to be painted is generally plated with chromium.
Stainless steel (which is essentially steel mixed with chromium) can be polished to a shine and will hold the shine, but it's heavy and its properties as a metal are not ideal for major structural pieces (though it does make pretty good screws and the like).
Bare aluminum isn't really all that shiny, but rather has a matte finish due to being "anodized". If aluminum is not anodized it also oxidizes and turns kind of a mottled white/gray.
Titanium can be polished to a shine and is reasonably corrosion resistant (it oxidizes but the oxide is protective). It also costs an arm and is only found in expensive bikes.
You can attempt to polish the steel and then clear-coat it, but you're not likely to be very successful. Steel that is to be exposed to reasonably harsh environmental conditions needs to be "primed" with a special type of paint that chemically bonds to the steel. Other coatings are apt separate and peel off.