A Primer on Primer

Before painting, primer is used.   It can clean up tiny surface imperfections like sanding marks but can also accentuate any nub marks left on a piece so nub marks should be sanded off well.  It's main purpose, however, is to prepare the surface for paint.  Paint does not stick well to the plastic, especially large flat pieces like those in gundam or tank models.

Primer also influences the colour that is painted over it so there are some basic rules I've come up with regarding primer color:

Military colours and dark colours - green, brown, dark blue, black:  Grey Primer.
Bright colours - red, yellow, light blue and green, pink:  White Primer
Metallic colours - silver variants (iron, steel, aluminium): Black Primer
Other metallics - gold,  brass: Silver Primer (haven't tried this one yet but soon).

There are additional colours for tanks such as sand or german grey that is a lot closer to the colour that will be painted over it.  The more a colour is different than the primer it's going over, the more paint you'll need to use.  I have created effects by painting a whole piece one colour but the different parts in it were primed different colours...in this case, I used gunmetal over black and grey and the difference in colour was quite a lot.

There's a few types of primer I use:

Vallejo Polyeurethane:   This comes pre-mixed and ready to spray which I love.   I have used grey, white, black, sand yellow and they're all great.  They spray MUCH better out of the .35mm Iwata airbrush I use than the .30mm Runway 13 one so I use the Iwata with this primer exclusively.

Vallejo primers come in 60ml bottles and very handy 200ml bottles with great spouts for pouring primer directly into the airbrush.  Shake them very well before use, especially the grey as it separates into a white and dark layer.

Mr Hobby (Gunze Sangyou or GSI Creos) surfacers/primers are just like Mr Hobby laquer paint.  They must be thinned for use.   They come in different 'thicknesses' which refers to the grit in the primer so higher numbers refer to a finer surfacer.  As you can see they have lots of products as well as Mr Surfacer such as Mr Primer/Surfacer (don't ask, I don't know the difference between Mr Surfacer 1000 and Mr Primer/Surfacer 1000), Mr Resin Primer (for resin models obviously) and Mr White Base which is simply Mr Surfacer in white.  These primers are of a very high quality with the obvious disadvantage that you need to mix them up first.

Just to add a note on Mr Surfacer.   Every time I use them I'm more impressed with the quality.  Vallejo primers seem like a tight layer of protection around the model which is good but these go on so well they become part of the model.  I haven't used them enough to differentiate between situations in which I'd use different grades of Mr Surfacer, so far I've only really used 1000, but as I use them more I'll get more of a feel for them.    As for mixing them, I mix them like I mix paint....add Mr Surfacer to thinner until it becomes the viscosity of milk...I use this method rather than measuring out amounts.  I think I tend to make my paint/primer too thin but I'd rather do that and have to layer the paint/primer on than make  it too thick and have airbrush issues. 

Alclad primers also come pre-thinned so are ready to use right out of the bottle.  Shake very well, there are some little bearings in there that you can hear rattling around to help.  I use black for silvery Alclad colours and grey for gold-type colours.   The middle bottle is enamel gloss black base for their high-shine colours.   Alclad primer, like their paint, is very thin.  I prefer to use my smaller nozzled airbrush (.30mm) for Alclad to decrease my paint use as it's so expensive and being thinned already you really go through the paint.   The primer is comparable to the finest Mr Surfacer as far as thickness goes.

 Gaianotes primers also have to be thinned, like Mr Hobby.  I rate their paint a bit higher quality than Gunze, but they're close.  I got these because Gunze didn't use to do colours, just the standard grey primer, but pretty much everyone does a grey, white and black one so the only really unique one here is the silver one.

As with all my paints, for thinning primers, I match the primer to the paint.   Gaianotes and Mr Hobby are both laquer based but I use Gaianotes thinner for Gaianotes and Mr Hobby thinner for their primer/surfacers.   Perhaps this is a  bit wasteful but in my experience you never go wrong using the primer that is meant for any paint/thinner.

As for enamels and acrylics I tend not to use them as primers.   Lacquer paint is so tough and durable that you can use any paint over it but with enamel and acrylic you have to watch out for other paints eating them.  I use enamel/acrylic paints as well as lacquer but always lacquer primer.