How to paint on glass

Glass paint is a transparent paint (with the exception of black and white which are opaque).
While there is water-based (acrylic) glass paint for children, artists usually buy solvent-based paints for a professional outcome. The solvent-based paints (such as "Vitrail") are durable after drying. The downside of solvent-based paints is the strong odour, which makes good room ventilation necessary while you are doing your artwork.

The paints are thinned and brushes cleaned with white spirit, - again a good reason for good ventilation by means of an open window or an extractor fan. Working outside is a nice option in beautiful weather.
Glass paint (as opposed to oil paint, acrylic paint, or watercolours) is very fluid and therefore needs to be contained within the intended painting areas on the glass. This is usually done using outliner which is acrylic paint out of a tube with a fine nozzle which helps to control the thickness of the line as well as the flow. Outliner comes in black, red, gold, silver, and lead-colour, all non-toxic and acrylic-based. The outlines have to be thoroughly dry before the design can be filled in with glass paints. Even after the painting is completed, the outlines will be susceptible to damage, and great care has to be taken when cleaning your glass artwork (- never use chemicals). Another possibility to contain the paint on the glass is the use of peel-off leading. The longest-lasting option though is using a self-adhesive lead-strip which you can bend into the required shapes and attach firmly to the glass surface using a pressing tool. The authentic look (or imitation of stained glass) is even more convincing if you attach the lead-strip on the reverse side of the glass as well. -A note of caution though: lead is toxic, so you need to always wash hands thoroughly before eating and even avoid touching your face or licking your fingers after you have handled lead! Very fine details within a design can be added lateron using a technical pen or a fine permanent marker.

Glass paints come in bottles. Please avoid shaking them, as it is difficult to remove the resulting air bubbles. Glass paints are intermixable only within their own range. That said, interesting effects can be seen when paints from different ranges of solvent-based glass paints are used together and refuse to mix completely. Also, mixing any colour with white on a palette will give you an opaque pastel effect which is quite attractive. But never try to mix water-based acrylic) glass paints with solvent-based glass paints, it just won't work.

The paints should be applied on a horizontal glass surface, or else they will run out of their defined outlined areas. You start by using a brush and carefully applying paint around the inside edge of the outlined area. Then you can load the brush liberally with paint from the bottle and while avoiding drops swamp the centre of the intended area with glass paint.

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Then the paint should be allowed to settle flat within the defined area, which will create the desired stained-glass effect. Continue by painting all the areas meant for that particular colour before you change to another colour. (Clean the brush with white spirit.) Be careful not to paint too close to a freshly painted area if you don't want different colours running into each other. Any mistakes can be corrected using a cotton bud though.

Oh, and always try to keep your paint bottles covered to avoid the paint going dry. By the way, it would be good to carefully stir paints that haven't been used for a while, because the pigment might have separated itself from the solvent. But again, avoid creating bubbles.

Paints can be diluted by adding clear glass varnish (solvent-based). This takes the intensity and denseness away. While painting, you can achieve various effects, for instance, little blobs of dark colour within a lighter colour can be blended in with a feathering movement of the brush.

When the painting is finished, it should be stored flat and covered with a dustfree box and left to dry for at least 8 hours; it is even safer to call it 24 hours or more, depending on the thickness of the paint.
Another note of caution: If you paint on flat glass sheets, please be very careful not to cut yourself on the edges. Either have them smoothed for safety by your glazier, or attach some tape around them. Also, if the glass needs to be cut to size, it is wise to do that before painting, not afterwards, to avoid disappointment.

It is also interesting to paint on glass items: bottles, plates, glasses, etc., but please remember that after painting these can be used only for decorative purposes. Wash them only in cold water, and dry them carefully with a soft cloth. Vases, bowls, glasses, and other such glass objects need to be painted and dried in sections, laid on an old towel or soft cushion and turned only when completely dry.
Once the artwork is finished and can be admired against the rays of the sun or even candlelight, you'll be pleased to have tried it and eager to progress to do more.