questions about painting

[gorbasho]

i dont know very much about paint. i plan to use testors for all my paint products(primer, clear coat, paint)and a airbrush. do i need to primer my yellow skyline if im painting it black? i read that you only need to primer if youre painting a dark surface. do i need to sand? what is the point of sanding? whats wrong with just painting over the original paint? would it still look good? also, should i use acrylic or enamel paint with my airbrush? can i use acrylic primer with enamel paint? is there such a thing as enamel primer?

[Horshu]

IMO, you should strip the paint (denatured alcohol will do it without harming the plastic....don't use mineral spirits), and use primer as a base. Primer binds to the plastic so that the paint can bind to the primer. Onepotential issue with painting over the paint is that you don't know what type of paint is already down. You have to be careful the order you lay down paint. Lacquer can go on primer, but if you were to lay down all 3 types of paint, you would have to do it in the following order due to the chemistry of the paint types: I personally don't sand primer, butI use Tamiya, which lays down very smoothly, but it's supposed to go: lacquers under enamel under acrylic. However, I was told by my local hobby shop that Testors enamel primer under Tamiya lacquer paint (Tamiya is having customs issues right now) is OK because that generally applies to hot lacquers. But try to stay consistent.
Now, real world, a friend of mine put Testors spray cans over stock paint, and had he used another can of clearcoat, it woudl have been a nearly flawless paint job, so it shouldn't crack or peel or anything.
BTW: If you're doing layers of a mica or metallic paint, don't sand between layers, as it swirls the particles. Load up on clearcoat (and learning the hard way, you really should wait a MINIMUM of 2 weeks (and I recommend a month...seriously) before wet sanding, polishing, and waxing the clearcoat. otherwise, your mirror-smooth paint job will get veeeeery tiny orange peel as it slowly settles (had a body shot owner tell me it would happen and I figured it wouldn't with model paints, but it did after a little over 2 weeks) Also, after 1 week, you can still put bits of finger prints in the paint. I highly recommend a month before sanding and polishing the clearcoat.

[gorbasho]

will this work:

1. sand off original paint, wash body, mask windows
2. lay down primer, wait 30 min.
3. lay down paint, wait 10-15 min, then another layer, wait 10-15 min, and maybe another layer?
4. lay down clear coat. how long do i wait after painting to apply clearcoat?

please tell me any changes to make in these steps

[Horshu]

I'd do 3 coats of primer. First coat is super-thin to where you can see specks of paint and some plastic. Wait 1/2 hour. Do it again. 1/2 and then do one more coat. Do an extra 2 coats of primer if you are constrained in a paint box and have to raise the sides of the car to get even coverage. Also, these times are what I've tested with Tamiya. Testor's primer is thicker and may take up to an hour. Let the primer dry a full day. If it looks rough at all, wait another day or two, give it a light sanding and rinse (the day or two of waiting is to ensure dry paint).
For base paint, you're right on what you're doing, except make sure those 3 layers are very thin and wait a full hour or more between first and second coats so that the first coat can seal the edges of your mask to prevent bleeding. After those 3 layers, wait 15 minutes and make the coats thicker (the wet coats). What you want is to get the paint looking wet but not dripping, so just get to the virge. Then let it dry until you see it start to orange peel or visibly dry and hit it again a little less than the first time (should be 15 minutes or so), getting that wet look again. You're aiming for keeping it saturated with paint for long enough to build a thick base but the key is not to put enough on to have paint drip (and you must be REAL careful of this when you clearcoat). Err on the side of less paint if unsure.
Treat clearcoat like another layer of paint, although some will say it dries differently and will crack as a result, but if you use the same brand as your base, it shouldn't be a problem. Do the same thing as you did with the base, although I like a lot of clearcoat, so I use 2-3 cans for a paint job. The more clearcoat (without having it run...it'll carry your base paint with it), the more leeway your have when wet sanding it smooth.

[gorbasho]

so if i didnt want to risk cracking, could i wait till the color coat dries completely? and what is wet sanding?

[Horshu]

Personally, I don't wait, since CC is just another layer of paint sans pigment, and I use the same brand, meaning it dries & shrinks as the same rate as the base color, but I've heard it recommended.
Wet sanding means applying wet sandpaper to your surface. I use micromesh sandcloth (goes up to 12000, but I wet sand 4k-8k, and 12000 really just shines it up when you wet sand), soak it in water, wrap it around a sanding block (dense sponge that came with the micromesh) and go up-down/left right a few times and then rinse. Wet sanding causes even adhesion between paper and surface plus it supposedly makes the sandpaper less abrasive. Because you're sanding clearcloat, if you see color (the paint be completely dry and safe from water), you need to stop and apply more clearcoat.

[gorbasho]

should i leave the windows masked when clear coating?

[Horshu]

I would actually clearcoat the windows personally so they get a more glassy look than black gloss will provide.