Gesso

Gesso is an artist's primer designed for preparing porous surfaces for paint. Obviously it's intended for canvas but it works equally well on any surface, including plywood, hardboard and Sundeala. It is available in black, and is a great surface preparation for model railways.

A standard technique in modelling is to prepare the model with a black or white base coat and then build up the colours from there. This is equally valid for railway modelling, even scenic elements, but the range of paint available is restricted: you can get black emulsion, but this tends to lift from plywood and can result in rocks and other scenic elements coming away from the baseboard complete with a patch of paint. Blackboard paint is relatively expensive, as is black wood paint. Gesso is relatively cheap, readily available in decent size tubs (a litre or more), can be applied with a brush, roller or pad, has excellent covering ability, dries quickly and presents a slightly keyed finish perfect for additional preparations including PVA, acrylic paint and the like; it is also very tough.

I use it for baseboard and wood preparation. I also use it lightly thinned for painting plaster mouldings.

The only caveat is that if you fail to clean your brushes before it goes off, the brushes are toast. I have yet to find anything that will rehabilitate a brush which has been left uncleaned with gesso on it. Before it cures, though, it is water-soluble and washes out of the brushes with soapy water.