Difference of paint vs. primer?

Non technical -

A primer is a specially formulated paint that is designed to be used on bare metal, wood, or other surfaces to seal the surface, increase adhesion, improve corrosion resistance, or ensure true color.

Different primers have different chemicals added to them for different purposes, such as in acid etching primers used in the automotive field to help stop rust. I'm not sure what type of lacquer paint Alclad is, but we know some lacquer paints eat or soften our plastic kits. So using a primer to seal it first is a wise idea, even if it is just another type of paint. Some people do use Krylon Interior/Exterior Primer or just their paint. It is an acrylic lacquer which doesn't seem to harm the plastic (recommended to test first).

Easiest explanation -
Think of primer as a thin coat of bondo or spot putty being sprayed on.
Think of paint as a thin colored layer of glass being sprayed on.

Sorry, best I could do at 3am. 

Ross, where are you when we need you?

Jim, I was installing windows—the real ones, not the operating system. Had that been the case, I still wouldn't be here…

Actually, your definition is very good. You directly or indirectly mentioned every function of a primer: surface sealing, increasing adhesion, corrosion resistance, increasing hide (true color), surface leveling, and protection of the substrate from the top coatings if they are incompatible with it (such as many lacquers and styrene.)

Except in specialized systems (such as Alclad) primers are most often flat because this promotes better bond to the top coat. In scale modeling, the most important functions of a primer are probably detection of surface imperfections and surface adhesion. With Alclad, the purpose of the primer is to protect the plastic surface and possibly to provide a very smooth surface and a certain amount of color enhancement.

Other than for specialty coatings like Alclad, you can use any flat paint (compatible with styrene) as a primer, provided that it adheres well to the plastic or resin of your kit. My favorite is Tamiya light grey acrylic.

Some people have reported problems with Alclad's primer. I suspect that these are due to use of incompatible solvents, but I haven't investigated the problem. However, this brings up an important point: whatever you use as a primer, if you thin it, be certain that the solvent you use as a thinner is completely compatible with the paint.

I hope you got the windows installed before the bugs got in Ross. 

Thanks, I tried my best in the wee hours... oh oh I did it again... sun is coming up.

Humm... I did forget to mention that flat paint is the choice for primer. 

Since my vapor inhaling days things have changed quite a bit. We only really had to worry about not putting lacquer on top of enamel and then urethanes came out. With all the different types of paint out there, how can we tell what can go on what without making a mess? I only knew I could use a certain can of Krylon primer because I tested it first on scrap. The only reason I even tried it is because I had some kicking around and didn't have any "model" primer to cover my spot putty body work. Just trial and error, pay attention to the forums or just stick to our old routine?

I'm investigating what paints my distributor carries and I might give acrylics a try soon.

Some people have reported problems with Alclad's primer. I suspect that these are due to use of incompatible solvents, but I haven't investigated the problem. However, this brings up an important point: whatever you use as a primer, if you thin it, be certain that the solvent you use as a thinner is completely compatible with the paint.

I don't think so.  I tried Alclad's primer several times.  It is very thin right from the can so no thinning is necessary.  I normally cleaned the surface with alcohol and allowed that do evaporate prior to painting, but even tried soapy water and just leaving the surface untouched to see if it made any difference.  Regardless of how I prepared the surface, the Alclad primer (the lacquer version) never seemed to cure.  I have a little 1/144 F-4 that I sprayed last year that still feels tacky (I keep it around just to see if it will ever get to the non-tacky-feeling point).  I finally gave up on it and just started using Model Master gloss black enamel and that works fine.

MusicCity wrote:

Some people have reported problems with Alclad's primer. I suspect that these are due to use of incompatible solvents, but I haven't investigated the problem. However, this brings up an important point: whatever you use as a primer, if you thin it, be certain that the solvent you use as a thinner is completely compatible with the paint. I don't think so.  I tried Alclad's primer several times.  It is very thin right from the can so no thinning is necessary.  I normally cleaned the surface with alcohol and allowed that do evaporate prior to painting, but even tried soapy water and just leaving the surface untouched to see if it made any difference.  Regardless of how I prepared the surface, the Alclad primer (the lacquer version) never seemed to cure.  I have a little 1/144 F-4 that I sprayed last year that still feels tacky (I keep it around just to see if it will ever get to the non-tacky-feeling point).  I finally gave up on it and just started using Model Master gloss black enamel and that works fine.

Coming from someone as knowing and experienced as you, Scott, that amounts to a pretty thorough recommendation not to use the stuff. I wonder what Alclad has to say about it. Has anyone tried to contact them about this?

jhande wrote:

…With all the different types of paint out there, how can we tell what can go on what without making a mess? I only knew I could use a certain can of Krylon primer because I tested it first on scrap. The only reason I even tried it is because I had some kicking around and didn't have any "model" primer to cover my spot putty body work. Just trial and error, pay attention to the forums or just stick to our old routine?

Trial and error—testing on a piece of scrap—is still the best way. While I may know enough to make an educated decision, I still do it that way. The reason is that paint formulations do change, sometimes very rapidly, in response to market conditions and safety regulations. This isn't just a headache for consumers—a manufacturer can be surprised with thousands of gallons of unusable paint because one resin supplier's product is slightly different from the same resin bought from a different manufacturer.

So a quick test shot on scrap or a paint hulk is always wise, even for "old routines" that always worked before.

Triarius wrote:

jhande wrote: …or just stick to our old routine? So a quick test shot on scrap or a paint hulk is always wise, even for "old routines" that always worked before.

Thanks Ross, that's what I though, stick to my old routine.

Test the primer on some scrap, wait a day or two for it to dry then spray a coat of paint, let that dry for a few days to see how it all reacts together.

No such thing as a short cut LOL. 

Coming from someone as knowing and experienced as you, Scott, that amounts to a pretty thorough recommendation not to use the stuff. I wonder what Alclad has to say about it. Has anyone tried to contact them about this?

I suspect it's something I'm doing wrong, but have no idea what.  It wasn't an isolated incident either, that happened every time I used it.  I got tired of tacky-feeling primer and Alclad finishes with fingerprints showing through so I just gave up on it.

I didn't try to contact them, probably should have but I didn't.  I've heard of others having the same problem though, so it isn't limited to just me.

MusicCity wrote:

I don't think so.  I tried Alclad's primer several times.  It is very thin right from the can so no thinning is necessary.  I normally cleaned the surface with alcohol and allowed that do evaporate prior to painting, but even tried soapy water and just leaving the surface untouched to see if it made any difference.  Regardless of how I prepared the surface, the Alclad primer (the lacquer version) never seemed to cure.  I have a little 1/144 F-4 that I sprayed last year that still feels tacky (I keep it around just to see if it will ever get to the non-tacky-feeling point).  I finally gave up on it and just started using Model Master gloss black enamel and that works fine.

Funny, I have been using Alclad primer on every model for three years and never had a problem. No thinning either, straight from the can. I do take a metal rod and stir up the pigment from the bottom first though. Shaking doesn't seem to mix it at all. Or maybe it's just luck.

Funny, I have been using Alclad primer on every model for three years and never had a problem. No thinning either, straight from the can. I do take a metal rod and stir up the pigment from the bottom first though. Shaking doesn't seem to mix it at all. Or maybe it's just luck.

That could be my problem.  I only shake the can, but I do shake it thoroughly and "Vigorously".  The last time I used it I was already mad at something or other and using primer that I knew was never going to dry only mad me angrier so to say that I "Shook" the can would be an understatement ;)

As I said, it's probably just something I'm doing but I got tired of messing with it and gave up.  I may have to try stirring the goop in the bottom and see if that helps.  Thansk for the tip.