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tamiya lacquer spray and unprimed bodies

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I've heard people say you can spray (rattle can) Tamiya lacquer acrylics (color coat) directly on a plastic body, no primer.  Also that it helps to polish the body as well- rubbing compound ? or sanding cloth to what grit (6000, 8000, 12000)?  Any experience ??  Thanks in advance.

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In a word yes.  Would I do it.  No!  Some of what you suggest is good, but probably for the wrong reasons.  All paints need something to grab ahold of.  Some get it by attacking the surface with solvents to get a grip.  Others have to have scratches(AKA key) from sanding  to lock on.  Either way, what ever you do to improve adhesion of the paint is good. 

That is one of two reasons for primer.  All primers dry flat which gives your color coat something to get a good grip on.  The other reason for primers is they have a high level of solids(all paint are made up of three things, pigment, binder and solvent).  This makes it so you can't see through them.  Basic plastic is somewhat translucent and if you just polish it out it still looks like plastic in certain lights regardless of how shinny you get it. 

Sanding or polishing the surface is a good thing because it removes the surface imperfections and gives you an even surface to lay the paint on.  It also gets rid of surface contaminates which may react with your paint. 

Here is my prep.

1. Wash the plastic with a good grease cutting soap in warm water.  This gets rid of anything the manufacture may have left on the surface.

2.  Wet sand the surface with a series of finer sanding sticks to get rid of all the mold lines, sink marks and other general unevenness.  I generally start with 3200 grit and do a 4000 and 6000.  nothing any finer is needed at this point.

3. Clean the surface with 50/50 mix of distilled water and  91% alcohol and a soft microfiber towel.  This get rid of all the dust and preps the surface.

4.  Prime the surface with a fine white primer(gray if the plastic is white) and let it dry. 

5.  Wet sand very lightly with a 4000 grit sanding stick.  This will show you any high spots or parts that need work. 

6.  Once you have all the areas worked out, reprime with a very light coat of the original primer and set it aside for a couple of days.

7.  Now spray with several very fine coats of color.  Don't expect the first couple of coats to cover.  If they do, you are putting too much paint on.  Once you have the surface covered then set it aside for a couple of day.

8.  If you are going to clear coat, then again, light sanding with 4000 grit to get a key for the clear, a light pass with the alcohol water mix and clear it.

9.  Now you are ready to polish.  This time start with a 4000 grit followed by 6000, 8000, 12000.  Then use a little fine or superfine grit polishing compound and you should be fine.

This is how I do it.  It sounds like a lot of work, but I am finicky about my paint jobs.  You can do as much or as little as you want, but eventually you will find a method that works for you.

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I use as few of the paint layers as possible (to improve model's realism). I often shoot plastic-safe paints over clean bare plastic. I use organic-solvent-based paints like Testors or Tamiya sprays.  Those solvents do not craze plastic but have enough "bite" to firmly adhere to the plastic. Or whatever they have (as I think this "bite" think is just something modelers made up). The binder material in the paint bonds to the model's surface on a molecular or even atomic level. No solvent "bite" required.  If you think about it, even the shiniest surface is really rough when you look at it under extreme magnification. That is where the bonding occurs.

Primers do have their place when the painted surface will be made up from multiple materials (such as mix of plastic, putty, or resin parts). Then the primer coat provides a uniform surface for the paint over all those materials.

Edited by peteski
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Sometimes I don't prime. It depends on the final color and how translucent the bare plastic is. Primer as used by most modelers does nothing to help key the color coat. most polish it prior to paint.

I did learn the hard way that two sided parts like dump truck beds need primed when painted light colors.

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