I've applied Vallejo Model Color over both Mr. Surfacer and Tamiya primer, but after a week it can still be rubbed off. What gives?

How did you "rub" it off? I'm assuming you shook the bottle really really well before using.

Airbrush or brushed? Thinned with water? It may not have been mixed properly or thinned with too much water. One other thing comes to mind is the first coat went on before the primer was cured.

Kimmo

I too found that it could be "melted" off with sweaty hands....LOL.... preparation is key, and sealing it after with something will go a long way... dont mess with thinners, just use modelair. If you do thin, use Vallejo thinning medium. Patience and practice.....

Hand brushed and thinned using Vallejo thinner.

TEST.
Mount the figure on a metal pin/needle..using heat & stick it in the bottom of his boot. Mound on styrofoam & cork square. Put on rubber gloves , do not touch the figure with your bare fingers..you could leave your bodies own oil on the figure which will not let the paint adhere.Wash the figure with soap & warm water, let it dry.Paint your figure with vallejo acrylic thinned with water ( not too much , you just need a little for brush work)..paint his legs. Thin some with vallejo thinner, paint his upper torso...let it dry for an hour , see if it rubs off. If not , good..if it does..proceed.
Do the same process, this time prime it with enamel primer, let it cure for 3 days. Do not touch it with your fingers.Then use your vallejo acrylic.
Then test to see if it adhere's better.
If it does , matt cote it.

Thin layer....wait to dry....thin layer....wait to dry....

Did you varnish it?

Once you are done painting, you need to seal it with a varnish.

Indeed.

Do you paint flat surfaces or details? How hard is it to rub off?
Few possible reasons:
-the thinner amount varies from bottle to bottle or by certain colors, some bottles are even unusable. Did you notice the paint doesn't cover properly?
-some colors doesn't hold that well, do you have the same issue with different colors?
-applying multiple thinned layers, working longer on the same area can soak and soften previous layers
-the already mentioned shake, stir, thinning issues, have you tried water?

I use water, personally never wash figures, and never use varnish (that can change colors), handle parts a lot by hand before, during and after painting, never had any suprise, of course paint will rub off after a while, starting at edges, or some sheen can show up.
The official curing time is 3 weeks, i get very solid, durable layers, when parts are put aside for longer periods, then handled by hand.
As suggested, do some tests. Good luck!

Does it have to be Vallejo brand varnish or will any do?

I think all varnishes will work on Vallejo acrylics.

I've had the same problem before when painting figures. Even though the paints been dry for a couple of days it rubs off (usually their boots as that's what I hold when painting them) so these days I always give them a coat of Vallejo Matte Medium 540 which stops it. Even if they don't need matting down I do it as a matter of course now as I found that when placing them in a dio before I'd have to touch the paint up as heat and sweat on my fingers would rub it off.

Have you tried using Valleyo primer? I had a similar problem and after using their primer it was all good.

Vallejo seems to be a little hit-and-miss. Some of my paint jobs are at risk of chipping or easily rubbing off, some are literally rock solid, where they can hardly be scratched with tweezers. One thing to keep in mind, if you don't want to take risks, is that vallejo paints take a REALLY long time to fully cure and become rock solid. For example, make sure there is no paint smell at all.

I paint almost exclusively with Vallejo and, while I can see it happening, I haven't had this experience yet.

Couple of points. While I love their paint range, I am not a great fan of their spray primers. I find the pigment a little on the heavy side. I don't like Tamiya primers at all - they have a tendency, in my experience to not adhere particularly well.

As to first coat primers, I use ONLY Citadel from Games Workshop. It dries matt, covers well on metal, resin and styrene. It's very good value indeed and due to its incredibly fine pigment, covers evenly. I'd suggest changing primers and considering Citadel for both primers and varnish. Also, they've recently added a médium grey to their White and Black shades....