Am I the only one...

Would I be correct in saying you have laid down a primer coat first????

With whites etc, you really have to give the colour something to contrast themsleves against.
Whites will not cover areas that are not primed correctly, and the more you try and force them to, the more it will run away, leaving your flat surfaces just as you described.

Prime with a similar shade, any primer in light gray will work, allow that to cure, then apply your whites.

Also check your thinner ratio, part of your problem could be your paint is too thin.
I am assuming your airbrushing.

I would concur with tweety above. You can cover white with black in one easy coat, but try covering black with white and it will take at least three coats before things start to look right...and then it may take a few more coats to look perfect!

A good light-colored primer is the key. I wouldn't go with white primer (although I use this all the time for colored topcoats), but with a very light grey as suggested.

I agree with the others. I use MM acrylics a lot, and as long as there is not a huge color difference I haven't had any problems getting white or gray to cover well. I usually use white or gray primer which helps. Also, as Sean mentioned, your paint may be too thin.

Phil must have been reading my mind, because not ten minutes ago I just finished swearing at a bottle of light ghost gray from MM. It's not that i'm too lazy to use primer coats, but in the scales we work in, I want as little paint as possible on my model. But, in some cases, such as this, primer might just be mandatory. But if you notice, the more coats of paint that get on a model, including the clear coats, the softer the detail gets, and I really hate that look. Who was it? Xtracolor? Some company originally came out with the idea that all their colors, even their armor colors, would be glossy, in order to save that step involving spraying clear to keep the decals from silvering. I thought it was a great idea, but apparently nobody else did, because it seems to have died quietly, like the elusive extruded strut stock that was so nice to have around.
Tom

Normally I do prime with a real light gray or some thing similar and have never had any problems(when using enamel). This time however the plastic itself is so light that the primer seemed superfluous. As for thinning, I was using it straight out of the bottle (with a brush, I'm to much of a piker to airbrush wheel wells in 72).I don't know, maybe I'll try priming it anyway. Maybe I'll switch back to enamel.

QUOTE: Xtracolor? Some company originally came out with the idea that all their colors, even their armor colors, would be glossy, in order to save that step involving spraying clear to keep the decals from silvering. I thought it was a great idea, but apparently nobody else did, because it seems to have died quietly

Well it hasn't died actually... True, most other paint manufacturers haven't offered their range of gloss colors the way Xtracolor did, but AFAIK Xtracolor enamel range is still in regular production. Not only that, recently they also launched their Xtracrylix (sp?) range - same idea, only that these are acrylic paints. I've read one or two reviews on the web and these new paints were highly praised if I remember correctly.
look at hannants.co.uk
(Hannants is the company that produces Xtracolor & Xtracrylix paints, besides being a major mailorder shop)

I have some limited experience using Xtracolor enamels (RLM78 and 79, relatively light colors) and was impressed how easy the airbrushing was (didn't even used primer, although the plastic itself was light grey already)

When it comes to colors like white, red or yellow I use Gunze Sangyo's Base white first.

Works great and on a few kits I went solely with the base white and no color over it.