I've used Wilkos primer with no problems, I think the trick with spray paint is to spray from farther away from the surface you're painting than you think you should -about 18 to 24 inches or so (basically, if you hold the kit outstretched in one hand, the spraycan would be about level with your head - essentially arms length distance).
Don't be afraid to lay the paint down too thin - you can add more later. Always keep the spraycan moving - up & down, side to side - so long as it is moving smoothly (think of your arm like one of those spraypainting robots they use for car bodies).
Get one of those pistol grip spraycan handles - it makes it a little easier on your fingertips!
For a really great matt black finish, I'd recommend the Plasti-Kote Barbecue spraypaint.
Its a flat, matt black, a little thicker than regular matt, it tends to leave a 'dust' effect that buffs up well (soft clothes brush is great! Also kitchen roll) to look like the slightly worn, holster-polished black finish used on real-world weapons. To my eye, anyway.
This is a resin Webley, painted with BBQ paint, buffed then drybrushed with a couple of different shades of silver, gunmetal and chrome acrylic model paints:
This is a Doopydoo E11 kit on 40mm OD aluminium pipe, painted the same way:
Excuse the faded pics, these are from a while ago!
The trick I found with all black guns & such is to get different paints for the different surface textures. For example, both the above kits have the grips masked off and sprayed with acrylic Satin Black for that plastic-y look.
Any real metal pieces (trigger guard on the E11 custom) don't get primered - the wearing of the metal looks better without it.
Even with a black gun, thinned washes of brown & black can give the effect of engrained dirt & grease - guns aren't self cleaning! The benefit of the rough texture of the BBQ paint is that dry brushing & dirtying down works really well.
Games Workshop do a fantastic range of acrylic model paints in various metallic shades for aging effects, plus they sell a clear coat in satin that if sprayed lightly is pretty flat.
Doopydoo parts are generally pretty sharp, but need a bit of cleanup, usually in places you won't notice too much, but if you can clean 'em up well before painting, it won't bug you later that you didn't fix it earlier (for example, drilling out the holes cleanly in the E11 folding stock, replacing molded-in screws with real ones).
Lightly sand the kit pieces, and also wash them in soapy water before you primer - use a fine sanding block (those sponge ones are great) for the biggest parts of it, then change over to sheet sandpaper for smaller areas & pieces.
Plastic/sponge pan scourers also work (try to get the white ones!), but can be a bit too soft (it depends on how hard the resin is - Doopy's resin is pretty hard, but if you were working on a toy gun paintup, the scourer would probably be enough to healp the paint 'key' to the plastic.
Hope that helps!
Coz.
