I found this to be a very informative article. I have had all of the problems talked about above and always blamed it solely on temperature or humididty. It looks like I was only partially right. Shaking the can not-enough and too-much looks like they can have more disastrous results that what I was thinking.
This isn't the first Friday Quick Tip about primer and it probably won't be the last. There's been some discussion on primer in our local community and it inspired me to post a few tips on fixing problems you are having with spray primer.
First, make sure you are starting with a primer, not a spray paint. Paint is not primer and there are some hobby primers out there that are pretty inexpensive. No reason to use the wrong tool for the job.
Once you've got a proper primer, pretty much any spray primer can give you good results as long as you use it properly and in the right conditions. The expensive spray primers are a lot more forgiving than the cheap ones, but if you know how to use the particular brand (or don't mind diagnosing the problem and fixing your technique) then you can get good results with any hobby primer.
Before we start looking at some pictures, let's consider the anatomy of the spray can. The nozzle is connected to a tube that runs down to the bottom of the can. That's where the paint and solvent are located (with a little propellant mixed in) and the rest of the can is taken up by propellant. When you turn the can and spray upside down to clear the nozzle, you're blowing propellant out. A lot of times the propellant is also a solvent, so keep that in mind when working with the can sideways to reach into those tight areas.
When you shake the can you don't need to go nuts, since you're just mixing the solvent and paint that may have separated. If you shake too much you'll agitate more propellant into the solvent/paint mix and you'll get a sputtering nozzle when you spray, which usually means splatter and an uneven coat. I never shake any primer more than ten seconds, even the Armory primer that people tell me really needs to be shaken well. But be sure to shake a little as you go along, especially if you are spraying a large batch of models.
So now you're spraying your models and things are not coming out right. Here are a few common problems and how to fix them.
Oops, you didn't shake the can enough. With some primers this is really hard to do-- the model above was sprayed after not shaking the can at all, but some primers need a little more agitation. If you see shiny streaks on your first few models, but not the rest, then shake the can more. The tell tale signs of extra solvent in the mix are the shiny spots that look like they've been hit by an ink wash. You might also see some bubbling or blistering and an inconsistent coat. The paint and solvent isn't mixed enough and you're not getting a layer of primer that will stick well to the model. This problem often shows up if you are batch priming and have let the spray can sit too long between shaking.
Darn, too far away. Whenever you see this rough coat and feel a flaky, powdery texture you are holding the can too far away from the model. The problem is that different brands of spray paint like different distances. I've found that the Armory spray works best in close, probably 4"-6", and the Citadel spray likes to be a little but further away, maybe 8"-12". If you are seeing this nasty texture but spraying at your normal distance it could be a lack of humidity. If you switch brands of spray also be sure to do a few tests to make sure you are familiar with the ideal spray distance of the new brand. Either way, when you see results like above, move closer.
Ah, the model of the impatient sprayer. Caking and cracking usually mean that you are either too close or moving too slowly. Spray in short puffs, keeping the can moving and remember what they say about several light coats being better. Yeah, they aren't just saying it to take up more of your time. Sometimes you can see cracking or bubbling from over-shaking a can and getting too much propellant mixed into the can. If you suspect over-shaking, just put the can down for at least 5 minutes.
Every primer can give you good results if you take the time to figure it out. Just pick a primer that fits your budget, patience level or one that is available at your local store. You'll probably ruin a few models along the way but remember that priming is like any other painting skill: the more you work at it the better you get. Getting the priming done right will take practice but if you get good results it makes the rest of the painting job that much easier.
And besides, that cracked and detail-obscured Ork will make a great statue once you paint it with a stone effect.
There is only one place in this article that I disagree with. This image -
The article states that this is from not shaking the can enough. I have not ever had this happen to a plastic model but I have had it happen on metal models that have been sitting out in my garage when it is cold. It is even more pronounced when I spray them with a can of primer that has been sitting out there as well. So I would say that temperature can play a roll in this issue as well. It isn't that big of a deal when this happens though - it is easily repairable. Just wait until it dries and then hit it with another coat. Much easier to deal with than cracked or furry models.They also have another article up on using the cheap white paint from Wal-Mart as a primer. I have been using this same paint lately and have been having pretty good luck with it.
The Painting Corps has tons of great articles. You should definitely check them out if you found the above article useful.