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Should I use a primer or not?

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  • Member since
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  • From: Quebec, Canada
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Should I use a primer or not?
Posted by xploringrailroads on Thursday, January 21, 2021 7:21 PM

Hi everyone.

I would like to know what are the rules for using a primer.

1. When should I use a primer?

2. When should I not use a primer?

3. When should I use a black primer, a white primer, a grey primer or any other color?

4. Is it ok to paint your models directly without a primer?

Thanks for your help!

Tags: Painting , Primer

Stéphan

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Posted by Overmod on Friday, January 22, 2021 5:45 AM

I see this coming up in other threads.

Many plastic models still have parting agent on their part surfaces, and this is probably the greatest reason for 'painting difficulties' -- simply make a practice of washing the trees or parts in dilute detergent solution and thoroughly rinsing them, then avoid getting finger oil on surfaces that are to take paint.

Often a kit will be made of plastic that has the wrong visible surface finish, or the wrong translucency, to be used without painting.  For this I start with a plastic-compatible primer -- which I have found to be usually in the beige range of color.  Sometimes these are made with the correct 'additives' to increase a plastic's surface activity, or 'self-etch' the surface slightly to enhance mechanical bonding.  Those are probably preferable to 'automotive' primers, or the general sort of Rustoleum that is intended to cover up a rough surface, provide handy fish oil to try to stop surface rusting, etc.  For this model work you want as thin a prime coat as possible -- light blocking, if you need it, is better applied as its own layer, and from the inside.  For a concrete building a lighter, and in many cases a 'primer gray' color, would be a much better choice than dark on the outside.

In my opinion, the best technique with 'number of coats' is to use multiple very thin coat 'passes' to build up what is a thin, but consistent, paint film.  If you see tangible thickness increase, there may be better materials or technique or equipment to be used...

My advice, on no more than personal grounds, is to get a competent airbrush setup when you can afford it, and learn how to use it properly, then practice until you can.  Others here can guide you to the best modern choices ... and things to avoid.  Can you do good work without?  Of course.

And never spray-dullcote your windows, even to try to make them aged and dirty looking...

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Posted by azrail on Friday, January 22, 2021 10:48 AM

If you are doing orange or yellow colors, use a light gray or white primer first. Red colors, use a light gray primer

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Posted by jjdamnit on Friday, January 22, 2021 1:41 PM

Hello All,

xploringrailroads
I would like to know what are the rules for using a primer.

There are no "rules" just "guidelines."

Check out this thread...

Prepping Plastic Structures

From following your post on your paper mill build I understand your excitement to get started.
 
One observation I have is most modelers do the prep work before assembling.
 
This prep work includes any priming, if necessary, and perhaps some basic painting.
 
I learned this in my early years of assembling 1/35 scale military models in the 1970s.
 
Hope this helps.

"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"

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Posted by snjroy on Friday, January 22, 2021 2:23 PM

Here are my two cents worth:

1. When to use a primer? my answer:  1) anything that regular paints do not adhere well to: metals, smooth plastics, engineering plastic. 2) anything that will involve rough handling later on, like a rollling stock shell. 
2. When to not use a primer?  Perhaps the question should be: when is a primer unecessary. My answer: Really small parts, styrene and other regular plastics. 
3. When should I use a black primer, a white primer, a grey primer or any other color? My answer: if your ultimate color will be on the pale side, then use white or grey. Black primer is probably better for steam engines.
4. Is it ok to paint models directly without a primer? Answer: some high quality paints like Polly scale (RIP), Protopaint and Acryl are pretty good at adhering to pretty much anything (brass would be the exception). Cannot say anything about solvent paints, I don't use them.
Simon
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Posted by SeeYou190 on Friday, January 22, 2021 7:09 PM

When should I use a primer?

I prime everything.

 

When should I not use a primer?

I don't know.

 

When should I use a black primer, a white primer, a grey primer or any other color?

If the model is to be brush painted, I prime black. I usually prime white for items that will be airbrushed. Sometimes I do crazy things with undercoating. 

 

Is it ok to paint your models directly without a primer?

Yes, many people do this with great results.

 

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by FRRYKid on Saturday, January 23, 2021 1:12 AM

This is my two cents worth:

I will start by saying that I don't use primers when I'm brush painting. (I don't use an airbrush.)

There are times even with primers you might need a second coat. (Think like painting a real world structure.) If the color change isn't dramatic, e.g. light color over light color or dark color over dark color one coat should do it. However, if you're making a dramatic change e.g. bare styrene/light surface to a dark color or dark color to light color, then two coats are probably needed. (Primer on the dark to light helps somewhat.) Even primers in the real world don't guarantee one coat coverage on dark colors. It is the nature of the paint formulation needed to get the dark/intense colors. I was on the job trained in a paint department and the knowledge still comes in handy to this day. I still help with paint as needed.

The only time I've had a major problem with coverage is with a certain line of paint that I won't name here. (To the best of my knowledge it is still around.) But I just know I won't buy that line of paint.

"The only stupid question is the unasked question."
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Posted by danno54 on Saturday, January 23, 2021 6:16 AM

If I'm painting over any sanded area, putty, glue or other dissimilar material I use a sealer-primer to get a consistant finish. I always use an airbrush for prime and finish coats. Too much of the fine detail in HO scale gets buried by the heavy coats from a rattle can.

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Posted by SeeYou190 on Saturday, January 23, 2021 11:09 AM

danno54
If I'm painting over any sanded area, putty, glue or other dissimilar material I use a sealer-primer to get a consistant finish.

This is an excellent point.

Priming over a surface with putty or filler material is important. This is especially important on model structures that contain plastic, wooden, and plaster parts.

I once built a wooden kit with resin detail parts and the final finish between the two materials was unacceptable.

-Kevin

Wink Happily modeling my STRATTON & GILLETTE RAILROAD. A Class A line located in a personal fantasy world of semi-plausible nonsense on Tuesday, August 3rd, 1954.

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Posted by kasskaboose on Saturday, January 23, 2021 2:24 PM

Something not mentioned is whether you use a color primer or standard white one.  I tend to use two coats of paint for most everything (e.g., masonite, stryene, plastic structures, figures, etc.).  Two coats works and since not going from one type of sheen to another (e.g., eggshell to semi-gloss), not worried about making it perfect. 

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Posted by MisterBeasley on Saturday, January 23, 2021 2:47 PM

Also, remember that primer is actually another layer of paint, so if you're concerned about obscuring fine surface details, you should think about whether a primer would improve the model or actually detract from it.

It takes an iron man to play with a toy iron horse. 

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Posted by Overmod on Saturday, January 23, 2021 6:47 PM

Something that might be of value here:  A primer is usually intended not only to cover dissimilar surfaces with a coherent coat, but to improve the chemical bond between the paint and the 'substrate'.  While there are a number of methods specialty primers use to achieve this, it can also be produced between various kinds of plastic itself and paint.

You can see examples of this in plastic bottles that have adhesive labels or some kind of ink or paint printing on them; some of the more recent ones are immensely tenacious if you want to remove the label for 'adaptive reuse'.

A search on 'surface activation energy' or 'adhesion promoters' will rapidly reveal a number of techniques with great applicability to model railroading where very fine surface detail has to be overpainted without compromise.  Several of these may be of interest as they are relatively easy to experiment with or use, and they eliminate the need for a 'coat' of material entirely in making a surface that takes paint.

One good approach is to soak the plastic parts in a strong base, like NaOH, for a couple of minutes after detergent-cleaning any parting agent -- in fact, I suspect some of these bases will themselves serve as 'cleaning agents' to help in that purpose.  (TSP in particular, although I have not found reference to using it in this context, should work if phosphoric acid does.)  This breaks some of the surface polymer bonds and adds OH radicals to them, which are highly reactive in bonding.  It is also possible to use certain acids (HCl, lactic and glycolic, and interestingly, phosphoric acid (as in Coca-Cola) for the same purpose, although the literature I have read says these take longer.  An alternative is to use an amine-bearing material (1,6 hexanediamine being a cited example) which treats the surface.  

An alternative treatment used frequently in industry is plasma 'etching' (which does have some mechanical bond promotion, but the primary effect can be enhanced bonding as important here).  A comparatively short exposure to RF-induced plasma including reactive species (oxygen being a major one) will break only the surface bonds; a secondary effect is to expose carbon at the surface that will covalently bond to materials -- this is one way PTFE can be attached to materials, if you were wondering.  In the model world, this reference might be of some interest, as its methods and equipment could easily be built in a 'home' environment from conveniently available materials...

There is some discussion how long the 'cleaned' activated surface can last without being promptly coated.  I suspect a good 'general rule' is to clean and treat immediately before painting -- budget time and materials accordingly.

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