I generally use a light gray primer (Walmart spray can) on plastic unpainted surfaces, both new and old stock where I have removed the original paint, before repainting.
Recently, I picked up a painted Atlas diesel- almost new- at a local swap meet. When I removed the factory paint with 91 alcohol, I found no primer beneath.
This made me stop and think- do I really need to continue my standardprocedure of priming first, or is there a difference in the paint process as applied in factory production- either in terms of the process itself or the manner of paint application which allows manufacturers to skip priming the bare plastic?
On plastic, to which paint sticks readily, unlike metal which repels paint, primer serves to cover the underlying material, old paint job etc. The lighter colors, yellow especially, don’t cover all that well. The underlying colors may show thru a single coat of yellow. The light gray primer covers thoroughly, and the yellow will cover the gray in one coat. For darker colors, use a dark gray primer, under red, use a red primer. Krylon and Rustoleum sell “auto body primers” in light gray, dark gray, and red. They stick to anything, cover anything, and present a dead flat paint loving surface that just hugs the top coat and loves it.
I do most of my painting with rattle cans. I’ll prime under the more difficult top coats, but I have had good luck with colors like white and blue sprayed right over the old paint.
It depends with Scalecoat II it is not recommended to use primer, although I will sometimes paint with a gray coat first pn black or dark red plastic for painting white, orange or yellow so I can get better coverage. I have started doing the same with my old Floquil as their Primer is no longer available and I have used all mine up.
Traditional reasons for using primer on plastic are: to protect it from the color paint that may attack plastic (think Floquil paints that used to be so popular), or in some cases to provide a base for the color so it looks right. ie putting a light color on a back shell may benefit with a coat of light gray primer for example.
You do not have to use primer on platic models if the paint doesn’t attack the plastic or it covers evenly and well on the plastic shell.
The only time I ever use primer is on metal. I don’t feel there is any need for it. Primer is another coat of something that covers the details. I realize that my opinion is almost diameterically opposed to almost everything ever said; or, written about on how to paint; but, this has worked just fine for me.
I fully agree…I haven’t use primer for years-more like three or four decades.
I freelance short lines so,more then likely I will just paint over the factory paint…
I had a Atlas RS1 I painted for my old Cincinnati Belt Line you could see the old lettering under certain lighting conditions…It gave the engine a touch of reality since a lot of short lines and terminal roads buys use engines and just paints over the former owners paint scheme-saves labor costs since there is no paint stripping involved…
I agree… primer on metal (sometimes wood) but not plastic. I have an additional reason for not using it, that being that it can contribute to covering up details.
You’re not alone on that NP2626! I agreee wholeheartedly. I remember removing the paint from the shell of an old BB SW7 (SW9?) and was amazed at not only how thick the paint was but how much detail was covered up by the paint. The new paint job was applied in two very thin coats and the details stand out very nicely. I have never had to primer anything but metal.
In the case of plastic locomotive or car shells, if the stripping process was successful and the shell is a nuetral gray color and no bodywork is required, applying primer is usually not necessary.
The usual exception would be in the case in which the bare shell’s surface is a dark color that would “bleed through” and negatively influence lighter colors applied over it.
I agree that, if not applied in thin smooth coats, primer can hide details on a model. However, in cases of primer being accidentally applied too thick, careful wet-sanding with a gray 3M Scotch pad can still yield a very smooth surface (yes, been there done that).