If you've just bought (or already have) a bicycle that has rust on it and needs painting, you may want to continue reading. Rust is less of a problem with newer bicycles, since many newer bicycles can't rust since they are made of aluminum alloy, stainless steel, and plastic. Also some bicycles may have non-rust frames but have screws and some other components of steel which may rust. If you don't plan to paint the frame skip to Paints to find out about what paint to use to paint components.
High quality bicycles frames are often well protected from rust and seldom need much painting. The highest quality paint job may be obtained by completely disassembling the bicycle and taking the frame to a shop that can paint bicycle frames. Some can apply the paint electrostatically and use a powder coating. The result is often a better paint job than the bicycle had when it was new.
But this is a lot of work and money. Another way that will likely take less time and cost far less is to simply paint it yourself with a brush. The paint job will not be nearly as good nor will it last as long but it's usually better than doing nothing.
Of course neglecting painting has one advantage: The bicycle is less likely to be stolen. If the paint is merely faded or an undercoat is showing thru in spots (or even almost totally), it may be best not to paint it. But if there is significant rust, painting will help stop further deterioration. In extreme cases, rust could cause something to break and result in an accident.
For brushing, you want a small can of paint (a half-pint is more than enough to paint the frame). For painting over old paint, almost any paint will do. But for painting over bare metal or rust you must use special types of paint. For bare metal, use a primer. Zinc Chromate is good if the metal is only slightly rusted. There is no paint for painting over heavy rust but you may sand the heavy rust to make it light rust. For moderate or light rust there are:
Rusty metal primers and rust reformers are only for a primer coat. They should be painted over with a top coat of ordinary paint. The rusty metal primer coat is soft and may absorb moisture but even if you neglect to put on a top coat it's still a lot better than doing nothing.
The quickest way to color-paint rusting spots is to use a colored paint designed to paint directly over light rust. Such paint (Rust-Mate) was formerly available in 1/2 pint sizes, but no longer. So now you must either buy a quart and perhaps have to deal with toxic waste disposal of the paint you don't use. Or you could buy a spray can of it which contains about the right amount of paint. There's also the problem of limited selection of colors.
Many paint stores (or paint departments of other stores) don't carry such specialized paints, so be sure to phone them, etc. before going there. Some automotive stores may have anti-rust paints. It seems that many paint stores don't have the paint-over-rust paints. Paints that claim to "prevent rust" are probably not suitable for painting over rust unless they say so on the can. But one person claimed that one may use half-pints of Rust-Oleum's metal paints for painting over very light rust. For a finish coat of paint, you might check hobby shops for small quantities of paint.
If you could easily remove all the rust, then you wouldn't need to bother about special paints for rust (but you would still need to use a primer for metal). However, removing all the rust may be too difficult to do. You should at least sand the rust spots some to remove all loose rust. A rusted surface is often pitted and to sand it smooth you would need to sand to the bottom of the pits which would also result in removing a lot of good metal. Sandpaper made for metal is best, especially the waterproof type for wet sanding. But a bicycle is relatively small so you will not be at much of a disadvantage if you use ordinary sandpaper. You only need sand where the paint is rusting or peeling.
An old bicycle (prior to 1970s ??) may have been painted with lead-based paint. So in this case take care not to breathe the paint dust from sanding or get the lead dust into the environment. If you wet sand, you could brush off the accumulated paint dust from the sandpaper into a bucket of water. Hold the sandpaper underwater and scrub it with the brush. Then when finished, pour off the water, leaving the lead paint dust on the bottom of the bucket. After letting it dry out, dispose of it as toxic waste.
If the bike is dirty, wash it with a rag. If you use detergent and water, be sure and rinse off the bike with a hose or wet rag afterwards. Rubbing the bike surface with liquid sandpaper will insure a better bond and can substitute for washing it.
For a bike frame, if there are too many spots that need painting, you can save both time by just painting the whole frame. It will also look better and there is no problem with color matching, since you can paint the bike a different color. But it's better to use a color close to the original just in case some of the new paint should eventually get scratched or peel, showing the old paint underneath.
For just painting the bad spots on the frame, it will look better if you have paint that matches the original. But do you care about looks? If you use another color for frame touch-up then it will look splotchy. But as a consolation, such a bicycle is less likely to be stolen.
You could get the paint custom mixed to closely match the frame color, but then the smallest amount of paint you can buy would probably be a whole quart.
In the 1980's Rust-Mate had various colors of paint for painting over rust in half-pint cans. It was sold at Standard Brands Paints. They had hundred of stores but they are now out of business. As a result of the loss of their main outlet for their paints, the Rust-Mate manufacturer, Zynolyte, discontinued the half-pints and eventually changed the name of the quart sizes to "Rust Master". The paint-over-rust paints available today likely are of superior quality to the old Rust-Mate. But they don't come in half-pint sizes. In the past, I noted some of Rust-Mate's colors fading more in the sun than other paints.