Is there anything specifically different about Primer?

[Bill Eh?]

I paint mostly with acrylics. Is there a specific component of acrylic primer which sets it apart from using, say a flat grey or flat white acrylic paint? Does anyone just prime with an acrylic (name your colour) flat coat before applying their acrylic colour coat?

My apologies for being lazy. I guess I just need to do a spoon test to find out the answer to this question.

I hope all of my fellow forum members are having a happy and restful holiday break.

[935k3]

In a word, sandability. Plain paints will not sand well. If you are using acrylics and don't need to sand they are OK. If you are using an airbrush take a look at Badger's Stynylrez primers.

 

[tom q vaxy]

my experience with Vallejo paints points toward adhesion. it's not hot enough to bite into the styrene surface by itself, so a primer gets over that hurdle. i'm not a big fan of V primer, but Tamiya is a good first coat. then do whatever pleases you. next 10 replies will tell you i'm wrong on all counts. 

[935k3]

4 hours ago, tom q vaxy said:

my experience with Vallejo paints points toward adhesion. it's not hot enough to bite into the styrene surface by itself, so a primer gets over that hurdle. i'm not a big fan of V primer, but Tamiya is a good first coat. then do whatever pleases you. next 10 replies will tell you i'm wrong on all counts. 

All good points. Acrylics do not adhere well and may lift if you have to do any masking.

[Pete J.]

All paints are made up of three things.  Pigments, binders and solvents.  Binders are the "glue" that holds the paint together and to the surface you are painting.  They are the predominate determinant in what the paint is called and what solvents are used to make them work.  The purpose of the paint is determined by the different chemicals and ratios use in them.

  I said this as a preamble to what makes primer primer. Primers have two purposes.  They provide key for your final color coats to stick to and to give you an even color surface to apply your color over.  Two parts of the paint have the most effect on that.  Most primers have a lot of pigment or solids in them so that unevenness of the base color  will not show or bleed through and give your final paint an uneven look.  They also have very aggressive binders to get a good grip(key) on the surface so the paint doesn't peel off. Some binders are actually designed to slightly dissolve the surface to get a better grip.  These are referred to as "etching" primers.  You will hear modelers talking about paint "crazing" the surface.  This is what is going on. 

So, a long road to the final, "Why prime?"  Primer is the final preparation for the surface to give you that even coat with just enough roughness for your paint to stick to.  

A couple of final thoughts about primer and it purpose.  A lot of us use primer as a guide coat.  We spray just enough on to cover everything and then sand the surface.  As you sand you will see certain areas that the paint comes off of first.  These are high spots.  You will also see spots that the paint is last to come off.  Those are low spots.  Thus, it "guides" you to spots than need to be sanded down or filled. Once you lay a light coat on and sand it off and it all comes off evenly, then you know you have smooth surface without any dipples or high spots or other irregularities.  Also, never use primer or paint for that matter, as a filler except for the finest of scratches.  It was not designed for that and will often "ghost" through with successive coats of paint. Filler or putty is for repairs, paint is not.  

I hope this gives you a better idea of why and how you should use primer.

Edited December 26, 2018 by Pete J.

[Bill Eh?]

Tom, Dale and Pete thank you for your detailed explanations. So, although flat paints have the required "tooth" for a colour coat, they unto themselves do not have the necessary adhesion characteristics required to sufficiently stick to the bare plastic surface. I will stick with purpose-made primers, before spraying any acrylic colour/top coats over them. I appreciate this forum for the generosity of the members in taking the time to share their knowledge.