Primer or no primer?

[FirstTimeModeler]

What exactly does it do, and when is it good to use?

[Khan]

Primer can accomplish a number of things. Its most basic purpose in any project is to provide a better surface for topcoats to adhere to the surface of whatever you're painting. For model kits the rule of thumb is that you'll need a "hotter" paint than your topcoats. Use enamels or acrylics over lacquer, acrylics over enamel. The solvents in lacquer-based paints can actually etch the plastic chemically on a microscopic level (too heavy a coat and/or too much solvent, and you can seriously mess up a project!). More to the point, by providing a more ideal surface, it primes the surface for painting, hence the name.

It also lets you find spots where you need more sanding or puttying, or other blemishes in the surface. For this reason it's especially important if you're doing a gloss or metallic finish. The last thing I can think of is that it can 1.) provide a base color that can affect the shade of your topcoats (some variantions on preshading use this, and it's essential for authentically reproducing auto finishes), or 2.) provide a neutral base color that will NOT affect the shade of your topcoats (For example, imagine a kit molded in obnoxious red or yellow plastic that you want to paint white or light gray).

It's not necessary, but it certainly helps. Enamels adhere to plastic pretty well on their own, so you can get away with passing it up if your finish is approximately the same color as your plastic. This also applies to certain brands of acrylics, but others can chip and flake something fierce if you don't prime with something better. That said, in our hobby, priming to check for rough spots is probably the most frequent reason to prime.

[Chuck1945]

The idea of 'priming' the surface can be over-rated if you use nothing but solvent based paints for your color coats since these will 'self-primer' ie, the first coat is also the primer.

However, as mentioned, regardless of your choice of color coats, priming WILL help detect any surface flaws before the color coat is applied. Also as mentioned, the choice of a primer color can help significantly in the coverage you obtain with your color coat. White, yellow, and red can be difficult colors to apply with the color saturation and coverage desired. However if a light neutral color (very light gray for instance or even white) is applied first as an undercoat, the color coat is much easier to apply.

Finally, acrylics lack the bite that solvent based paints have, so adhesion can be improved if you use a solvent based primer. Personally, I prefer to make the plastic itself extra clean to help with adhesion instead of using a solvent primer under acrylics, since the reason I choose to use acrylics is to avoid the use of solvent based paint.

[Edgar]

One bonus, which many forget, is that it will give you the same surface, over your entire model. Some fillers have a habit of soaking up the paint, a little, so that the patch, which you went to so much trouble to eradicate, shouts "Here I am," as it shows up as a different finish from all the rest. Every model, of mine, has a primer, no matter what.

Edgar

Edited December 7, 2006 by Edgar

[The Loggie]

I build mostly aircraft, and I rarely prime a model. I use only acrylic paints, sprayed-on. Then I spray or brush more over the top. I'll prime if I gotta do a light colour over a lot of dark filler (eg, Mr Surfacer).

I usually spray the whole model with the colour of the belly, mask the belly then spray the top. I reckon that I get away with it because the belly colour IS the primer coat.

George, out.................