Metalwork

Paint is flaking off back to the bare metal: This is normally caused by painting galvanized metal surface without first having applied a suitable zinc primer. To repair all the paint must be scraped off completely and the metal primed with a suitable compound such as Calcium Plombate.
I need to paint an old iron gate which is very rusty: Scrape off all loose rust and wire brush it thoroughly, then apply a coating such as Hammerite or special metal primer which bonds with the rust.
Brush marks look ropey or uneven: On large areas of metal, like on doors, where the surface is very smooth thin out the paint. If necessary giving two or three thin coats rather than one thick coat. This is usually only necessary when using primers or undercoat. Gloss and enamel finish paints are generally self-levelling.
How do I repair dents and scratches in metal surfaces: The best filler to use is a two pack filler such as people use on car bodywork. Apply slightly proud of the damage but as smoothly as possible. When dry, down with progressively finer grades of sandpaper.