I've run into this issue many many times over the years, and it happens for a number of reasons. Either I didn't clean and prep the surface properly, didn't apply the primer properly, or didn't choose the correct paint (see a trend here? Yeah, despite knowing better, I tend to get lazy and rush things sometimes).
With metal and plastic surfaces, it's usually not too common a problem. Most often, the project doesn't need much in the way of prep.. a well shaken can of primer (make sure you hear the metal mixing bead rattling around for at least 30 seconds) sprayed in good conditions (not too humid or cold) will do the trick. Don't overdo the primer either. Just a light coat, otherwise a thick build-up of primer will dry into a slick surface, giving your acrylic modelling paint very little texture to adhere to.
Resin or other materials can be a bit of a problem though. In the case of resin, it's a fairly porous material, and it tends to sponge up the oily release fluid that they coat the insides of the mold with. It some cases, it means that you need to soak the resin parts in a warm soapy solution for a good long time (overnight in some cases), then rinse them off thoroughly to get ride of any oil or soap residue. This does happen to plastic and metal parts on occasion (particularly the Privateer Press version of "plastic"... that sh*t is horrible stuff to work with, for a long list of reasons), but leftover oil residue is most commonly found on resin components. And it's worst on resin, because the oil really penetrates deep in some cases.
So what happens when you are happily chugging along, in full hobby mode, and your fully assembled model gets to the primer stage? Oftentimes the primer just won't stick. It'll pull up into puddles as it dries and the primer contracts. Sometimes it won't even happen at the primer stage either... oftentimes it'll happen at the paint stage instead.
What's happening is that the liquid content of the paint or primer is evaporating as it dries, which means that the volume of the paint decreases as it becomes a solid. Naturally, it becomes smaller, and ideally, dries to a nice super thin layer of paint. However, if the surface it's clinging to is too slick to get purchase on, it'll lose grip and either puddle up into spots of dry paint, or it'll look fine for now, but flake or rip off later on (likely while you are handling the model, or it's rubbing against the soft foam interior of a carrying case).
This is what happened to me recently:
Here I am, desperately trying to finish off a display base for a painting competition entry that was due the next day. The base platform was a simple picture frame that I had found at Ikea, to which I glued a thin sheet of torn up corkboard to. I hit the whole thing with a layer of black spray primer (standard GW primer), then airbrushed and drybrushed the cork, masked off some areas and airbrushed on some quick street markings (the white diamond denotes a High Occupancy Lane, which I thought appropriate for an Armoured Personnel Carrier... the minivan of tanks). I was just about the repaint the frame itself with some more black (to clean up all the overspray and overbrushing) when sections of the black primer just started coming off on my fingers.
The horror!
It appeared that the fake wooden frame (I think it was some sort of laminate surface) was simply too slick for the primer to get a good grip. While it initially appeared okay, it couldn't hold on while being handled by my Spiderman-like fingertips.
Crap. The painting contest was the very next day, and I was already working deep into the night. Hmmm... perhaps another hit of primer would do the trick... this time I would use a Vallejo airbrush-ready paint on primer. I could use just enough finesse with the airbrush as to avoid hitting the road bits.
Well, it worked... for a while. Then when I tried picking it up and getting back to work on it (adding oil spots and patches of pigment for weathering), this is what happened:
At this point, the black just started peeling off faster than a stripper's clothes under a rain of large denomination bills.
Aggghhh!!! My body and brain craved sleep in the worst possible way, and my heart was pounding under a barrage of stress. I was ready to grab the whole thing and hurl it into the trash bin (and then take a sledgehammer to the bin as well). But then I remembered a craft project that I had done with my then preschool age son a little while back...
We were using pieces of clear recycled plastic packaging to represent stained glass. The problem with the stuff is that it's perfectly smooth... absolutely no tooth for paint to adhere to. And you couldn't hit it with primer either... we wanted to retain the translucency of the clear plastic, and leave a coloured tint instead.
Paint straight up wouldn't work... the paint didn't even want to leave the brush for a quick visit to the plastic. Watering down the paint made things even worse. The surface was so hydrophobic that any liquid (no matter what consistency) would simply run off of it.
In the end, I ended up mixing some white glue and dish soap into the paint. Now, you might be thinking, "Say whaaaatt???" and I would totally understand. But hear me out before you close your browser and play more World of Warcraft instead.
Dish soap kills the surface tension of liquids. A teeny tiny amount of the stuff stopped the paint from contracting into puddles, and instead allowed it to dry as a smooth layer instead. The trick was not to use too much of the stuff... a fraction of a drop did the trick, so long as it was thoroughly mixed in. Perhaps the flow release acrylic medium you find at art stores works better, but dish soap worked alright. It works on a molecular level, and I studied English Lit instead of Science at University, so as far as I understand the principle, it works by magic. Yup, pure magic.
The white glue, on the other hand, lends some adhesive properties to the paint. Again, I don't know how (heck, my ancestors thought that ingesting ground up bull testicles helped old men make babies, by the same method of logic I was using, so yeah... it's all magic). A little bit more white glue was required as compared to the dish soap, but not much more. Again... it all required thorough mixing with the paint and soap.
The great thing about both dish soap and white glue is that they both dry completely clear. There is little to no pigment in either (unless you're using glitter glue, of course), so the paint is the only thing that determines the colour you are laying down.
So, after raiding the kitchen sink and my 5 year old son's craft supplies, I got to work. And you know what? The damn thing worked. I had to apply the paint by traditional stick brush, of course (it would have killed my airbrush), and it took damn near forever to dry properly (and a subsequent second coat as well, for good measure), but it worked like magic.
I like magic.
Anyway, finished pics:
Note that I worked the oil stains and weathering pigments over again with a soft brush and a little thinner to make them a bit more subtle.
And a pic of my finished entry, taken in my own figure case months after the painting competition:
Well, it won't win any Golden Demons (in fact, it was beaten by a very nicely weathered Leman Russ tank by Matthew Beavis... check out his blog entry regarding it here), but considering what I went through, and that it had started off looking like this:
I was happy with the final result.
In hindsight, I should have sanded the faux wooden frame a bit more aggressively. I did make a quick pass over it with light sandpaper, but it still didn't give it enough tooth and texture. And I've been told that GW primer is not a "true" primer, but the stuff has almost always worked for me before (I may give the latest generation of Privateer Press P3 primer a go next time, even though I had huge failures with the first generation of the stuff when it originally came out).
Most of all, I probably shouldn't have left the display base to the very last minute. You always have to give yourself enough time for fixing mistakes and the inevitable unforeseen challenges that arise while working on a project. In fact, I usually tell people that they should finish a competition entry at least a month ahead of time, put it away, and then look at it again a week before the competition. By then, you might have a fresh look at the model, see any flaws that need fixing, and still have enough time to correct them before entering it.
Of course, I never take my own advice, but other people sometimes do. Which is why I always seem to be the number one cause of my own defeat in painting competitions, but hey, nobody's perfect, right?
As for next year's rematch against Matthew and the other incredibly talented painters (more and more of them seem to be popping up all the time) in my local area, I know I have to push myself to a whole new level now. To be frank, I think Matthew's Leman Russ is no where close to what his current painting skill level is now. If you check out his blog, you'll see that he improves faster than anyone else I know. At the time, he was doing his best James Wappel impression, but he's now incorporated a number of other great artists talents and techniques into his arsenal of tricks (Meg Maples was teaching a course in Vancouver recently, and Matt just gobbled up her lessons like a Hungry Hungry Hippo). My Rhino was just as good as his Leman Russ, if not just slightly better (in my highly biased opinion), and I think part of the reason I was beat was because I entered something very similar the previous year (same judges, who are probably getting tired of seeing me enter red tanks year after year), and because Wappel's style was something new and fresh to them. Matthew's work was a bit rough and experimental at the time, but he's getting much more refined while still experimenting more than just about any other painter I know locally. It's a killer combination, and I'm going to have to really work to compete against it.
Bribes. I'll probably have to resort to bribing the judges. And maybe magic. If only I can find some magic ground up bull testicles to mix into my paint next time, I think I have a good shot at winning.
