Why does a wash remove paint to bare plastic?

[Norm]

Or perhaps I should say ‘how’ does this happen?

It is Something I have noticed lately and I think the common denominators are;

hard plastic 28’s (but pre washed in soapy water)

Primed with vallejo acrylic primer (this is very thin and can be used in airbrushes.

Block painted and then a wash applied from GW or army painter.

Once dry, I notice in minor areas of the figure that what was paint has now gone back to bare grey plastic. I am guessing this happens where the wash might collect (such as above belt equipment) and run down to (such as the bottoms of long coats). So I assume these are the places that the wash pools and takes longer to dry.

What is actually going on at the surface of the paint?

I can only assume that I need either thinner washes, a different primer or to varnish before washing.

[Daeothar]

Varnishing before the wash might be the way to go, obviously (with or without the issue you describe), but that does not take away the cause of the problem.

I'm thinking that there are two factors at play here.

Perhaps the pre-painting washing is not as effective as you thought, so perhaps change the type of soap? And do you let the minis dry to the air or do you dry them them with a (paper) towel? Because if the first, when it's not rinsed off well enough, soap may pool in exactly those areas and prevent any primer or paint to adhere to the plastic.

The other one is that maybe the medium of the wash is reacting with the paints underneath, perhaps because those layers were not yet fully dry?

[Jay Adan]

Are you sure that's what's happening? It sounds more to me like the wash itself is discoloring as it dries. this sometimes happens when the wash is particularly thick or has gone bad.

[Daeothar]

True; certain washes sometimes dry whitish. Apparently this has to do with the medium not being distributed uniformly throughout the wash (through shaking). But in my personal experience, this only happens when I thin the wash with water.

And then only a certain few Vallejo ones (especially the black and green). When I got myself a new bottle of black (with 'New Formula' on the label), the issue was gone...

[Jay Adan]

Okay, just checking. It just seems strange that an acrylic wash could re-wet and then strip two layers (one paint, one primer).  I can see a wash stripping off a previous layer of wash (because I've seen that happen) but that's easy to figure out when it happens.

[Norm]

Thanks, a few ideas for me to check there. Of interest is that some of the ‘spotting’ occured in the same place on different figures, so on a group of German WWII officers in long coats, I found that above and below the holster had been stripped of paint, as had around a bag hanging of the belt, as had one under cut under a belt and as had the top of the boots immediately under the coat (i.e. hidden from normal viewing angles, but there when you looked side on or from a lower perspective, or turning the figure upside down!).

Oh and the cuff of the coat!

I think I will try a rattle can primer and do everything else the same, just to see if that aspect of the process can be eliminated. I will also do two figures with a newer bottle of wash, between these two things I should be able to tighten up on the number of ponderables.

[Cosmotiger]

I've found that the Vallejo primer is not great. My guess is it isn't bonding sufficiently to the plastic, and the wash is scrubbing primer and paint off the model.

[SteveBurt]

I find artist's acrylics very good as primers for plastic figures - they make a good flexible layer

[Billchuck]

Thanks, a few ideas for me to check there. Of interest is that some of the ‘spotting’ occured in the same place on different figures, so on a group of German WWII officers in long coats, I found that above and below the holster had been stripped of paint, as had around a bag hanging of the belt, as had one under cut under a belt and as had the top of the boots immediately under the coat (i.e. hidden from normal viewing angles, but there when you looked side on or from a lower perspective, or turning the figure upside down!).

Oh and the cuff of the coat!

I think I will try a rattle can primer and do everything else the same, just to see if that aspect of the process can be eliminated. I will also do two figures with a newer bottle of wash, between these two things I should be able to tighten up on the number of ponderables.

This sounds like maybe you didn’t get primer into those low spots.

[westwaller]

Is it any thing to do with the primer not being fully cured/dry when the subsequent layer of paint was applied? I seem to remember that Vallejo primer does need to be left overnight before any further painting takes place.

[Norm]

Could be, I think in this case it was a rush job and I did not leave it, but usually I do and have still seen the same problem. What initially seemed like a simple problem, certainly has multiple dimensions to it.

[handle]

Do you think your wishes with water? And if so, do you get the same effect if you don’t thin the wash, or use something like Vallejo’s Thinner Medium?

I live in a hard water area and get a white-ish residue if I thin washes with water, so just wondering if it might be something like that.

[Norm]

I did use water with this wash. We get limescale and I never know whether that is hard or soft water that does that (is it acidic soft).

On my last project I mixed the wash with GW Lahmian Medium at a ratio of 1:2 (ink to medium) as the Vallejo washes are very strong. Though that time, the figures were metal and I used a Hammerite special metals primer, so with so many thing changing in that job, it is difficult to compare, but I don't recall losing any paint due to the wash.

I have been thinking about how to not let it happen, perhaps I should try to replicate it again across say 3 figures, with slightly different applications to each figure to identify the combination that DID do it.

[handle]

It’s a really curious one! Did you notice any residue around the grey area the last time you observed this, or were there just ‘bald’ patches left on the mini?

Also, just a thought - could you try scraping the grey patch with a scalpel to check that the paint really has been removed?

But your approach sounds good to me, trying to isolate the variables and then running some tests - I’d be really interested in the outcome if you have time to post a follow up.

[Ogrob]

This happened to me years ago! I think it was with Vallejos white primer in a regular size bottle. Switched to their Surface Primer range, the polyurethane ones, and have not had a problem since.