non-model primer

The pigments in hobby paints are sometimes ground more finely than say, automotive primer; and some hobby paints are formulated to form thinner coatings than paints intended for 1 to 1 scale objects. You may be able to find an automotive primer with similar characteristics.

Another thing  you can do is wet sand the coarser primer with fine grit—800 and finer. This can produce a very smooth surface.

If by Wal Mart stuff we're talking Krylon, then yeah, I use the stuff too. I use the white primer, makes the colors come out how I want them ^_^. 

How I do it is a decant them into a jar and air brush it on.  Gotta clean the airbrush VERY VERY VERY well after this, since if not, it's a pain in the bum to get all the dried paint out.  I usually gotta let it soak for a couple of days in laquer paint thinner if I don't clean it that well.

Anyways, my techinque is a couple of light coats with the airbrush, let dry over night.  The next day (sometimes I even wait two!), I take a toothbrush and go over each piece.  Gives it a nice smooth finish.  Then, I do another coat, let dry a day or two, repeat.  

Something I've come to notice on the stuff is that it don't adhere well on corners, it seems.  The stuff just chips right off, even if I just brush it with my finger.  That's why I do multiple coats, to make sure it sticks on the whole piece. 

EDIT: Oh, and I forgot the most important part: safety!  Look, with Tamiya, MM, and other acrylic paints you can get away with paintin in your garage or by an open window.  NOT WITH THIS STUFF.  This stuff reeks, stays in the air for a long time, and hurts you.  Put on a mask for using this stuff. 

Vinnems wrote:

Something I've come to notice on the stuff is that it don't adhere well on corners, it seems.  The stuff just chips right off, even if I just brush it with my finger.  That's why I do multiple coats, to make sure it sticks on the whole piece.

Either you are spraying too dry, or there is a surface contaminant in those areas.

I recommend not decanting aerosol paint. I used to help develop the stuff for a living—all the components of  a can of aerosol paint are carefully matched to optimize paint performance, including the valve and spray head. You will generally get as good or better results, and probably better adhesion, spraying from the aerosol can, rather than decanting it.

Decanting is also unsafe. God help you if there is a source of ignition anywhere near. The propellant in most aerosol paints is propane or sometimes butane…BOOM!

I feel your pain…

I just did an experiment: I sprayed a good quality automotive primer on a sheet of styrene and allowed it to cure. I then overcoated with Future. The original surface looked smooth as velvet, even under low powers of magnification. The Future brought out the surface "detail" Sanding with 800 and 1000 grit smoothed the paint surface, but I was surprised at how much sanding it took, how much paint I was able to remove, and how thick the coating was.

Jhande may correct me, but I think most automotive primers are intended to be sanded.

The fact of the matter is that many aerosol paints are formulated to produce relatively thick paint films. When you decant one, you are probably only spraying the finest pigment particles, the others having settled rapidly, so you get a thinner coating. 

Methinks some of us need to research a good, inexpensive primer that does not require the practice of decanting the material from the aerosol can. Someone pointed out that there are such things (I think Tamiya makes one.) but as usual, the price is a bit high for something that gets used in large quantities.

Unfortunately, I no longer have access to a paint lab, or I could do film thickness tests on people's suggestions. Sigh.