You will get the best finish if you do the prep. This is the key to a quality finish.
First remove all the paint and rust using paint stripper and a wire brush, then carefully de-burr fasteners as necessary (sometimes you will find a raised metal from tool slippage etc). I use a wire wheel on my bench grinder and an assortment of small wire brushes and tube brushes attached to the drill to get inside of bores such as nuts and spacers. A mild solution of Hydrochloric acid will remove rust and old plating, Citric acid will work as well. Be careful with high strength steels such as axles and pivot shafts, sometimes prolonged immersion will cause embrittlement so if in doubt leave these parts aside (most chassis and engine hardware and brackets, chain pullers and dust covers etc, will be fine).
Experiment with dwell time for zinc removal, it only takes a few minutes and you will see the fizzing occur soon after you place the parts in your acid. Rinse the parts thoroughly in fresh water, it does not matter if surface rust appears after the wash, the plater will process the parts before plating to prepare them. In fact some platers prefer a uniform film of oxidation. Over cautious "helpful" customers will coat the parts will any number of oily, waxy or silicone preservatives that the platers pre-treatment might not remove adequately.
At the very least, make sure you strip all the paint and clean the parts up as best you can. This will ensure the parts plate well.
