why use wood-primer?

[Kevin]

At the risk of being labelled a bodger :

Some time ago I needed to paint some newly fitted woodwork (probably skirting or architrave). I didn't have enough wood-primer, but knew I shouldn't go straight to gloss so in a hurry grabbed a pot of emulsion and used some of that as a base-coat.

It seemed to work fine to me and I've used it again a couple of times since, but each time I keep thinking, I really ought to be using wood- primer. So can anyone tell me why I shouldn't continue to use it. It is obviously cheaper than primer and more to the point I always seem to have a little left over.

By the way, I'm only talking about internal use here. I much more inclined to do it properly if it's going to be affected by the weather.

[Stuart Noble]

Primer is usually more opaque, so you need less of it to hide the original surface. In theory it's also tougher, being an acrylic rather than a pva resin. In practice I don't know that you could tell the difference in the final finish

[The Natural Philosopher]

emulsion is not a bad primer actually: arguably acrylic primer is just 'thick' emulsion, anyway

Primer is really there to be a layer that sticks to the wood, and to the overcoats. And maybe fill the grain a bit.

That's all it really does.

[Andy Dingley]

"Priming" is the process of applying something that sticks to the substrate, and that can be stuck to by the following coats.

So if you have a paint that's sticky and overcoatable, by all means use it. For surfaces that are easily stuck to, this is easy. For other surfaces, it gets harder and you will need to use dedicated primers. There are also materials like MDF that need a primer that's low in free water, otherwise they raise fibres.

Opacity comes from an undercoat that is rich in filler. At one time three coats were a minimum (primer, undercoat, cover) because it was impractical to make one paint do more than one task simultaneously. Nowadays combined primer-undercoats are pretty common. Smoothing a rough surface needs multiple coats and finish paints are more expensive, so there's always going to be a need for something cheaper to bulk up beneath. "One coat" paints sound great, until you need two coats anyway and find yourself using expensive paint for the undercoat.

Watch out for finishes that shrink on drying over chalky undercoats of low cohesive strength - you'll get crazing, maybe not for a couple of years.

[Rod]

Not only can you use emulsion, but for some substrates it is better than ordinary primers - e.g. hardboard.

In fact I would go further. On all our internal doors I have painted with several coats of emulsion, then finished off with a couple of coats of tough PVA varnish. (All done with rollers.) No problem whatsoever with strength and durability. Also, has not yellowed like gloss despite not having been redone for quite a few years.