Gesso (Paint on Primer)

Most of the time, you'll be spraying primer on your models. It goes on evenly, covers a large area quickly and is pretty cheap. However, sometimes you need to do a fix, touch areas up or add on a little something. You obviously can't spray primer on a model that's already being painted and painting without priming will cause problems down the line. Not only does the paint go on poorly, but it will come off easily. Some people use the new Citadel Foundation paints instead of priming, but even these will be apt to scratch off.

I can't remember where it was or who I was talking to that the word "Gesso" came up. [jes-oh] Gesso is simply the name of art quality paint on primer. It provides a good painting surface and goes on even smoother when mixed with a little water. A spray on primer will still be more even than a paint on one, but gesso is still great for touch ups and other times you don't want to spray. (or during the winter when it's too cold to properly prime miniatures) (such as in an area or during a season where weather hits the extremes as spray paints degrade greatly when it's too hot or too cold.)
Update: Reader Deadmeat points out that primer can become useless in heat extremes as well. 

• A single thick is enough for things that aren't very detailed. Gesso is usually thick though, so for detailed miniatures you'll want to paint a couple layers, each thinned with water.
• I've found White Gesso can be a little streaky, affecting the quality of your base layer. To avoid this, make sure you use long even strokes with a wide brush rather than a pointed one. Another way to avoid the streaks is to thin your gesso even more and use many layers.
• Depending on the paint you'll be layering first, you may not need a perfectly white base. The important thing is to create an even, non-streaky surface that won't come off easily and paint applies nicely to.
• If your first layer is going to be Citadel foundation paint, there's a really good chance you'll be fine with a thin layer of gesso.
• If you're working with lighter colours like yellow, it becomes more important to create a nice, even, strong white undercoat.

2 layers of gesso, thinned with water, to prime pewter.

Gesso is white and will cover even black just fine.

As with the rest of my art store products, I use Liquitex. I'm sure there's other fine brands out there, this is just what's available locally. http://www.liquitex.com/Products/surfprepgesso.cfm I'd recommend either ordering online or finding a local art store and seeing what they have. This is one of those things that you're better off going to where you'll get the better product - art supplies from art stores and tools from hardware stores rather than hobby/craft stores. (a bad primer can ruin your entire model)

+ Something gesso has over primer is you won't get a too thick base coat, since you're the one painting it on.
+ You also won't end up with a rough surface like you can get from spraying primer on in dusty environments or when it's too cold.
- Takes a few thinned coats.
- Can be streaky if you're not careful.

Interesting Uses

• As there's no chemicals, you can use it on problem surfaces such as styrofoam. (since the chemicals in spray on primer eat styrofoam for breakfast, second breakfast, elevenses, luncheon, tea, dinner and supper)
• Liquitex also carries a clear gesso which would allow you to paint or tint clear surfaces (clear or coloured film / plastic from audio stores, overheard paper, etc) for things like windows and stained glass. (Although overheard paper labelled to be used with inkjet printers already have a slightly rough absorbent side.)