mystery spots appear with Vallejo Model Color black acrylic paint

[builthatch]

the band across the windscreen is the problem. this is a 1/43 windscreen, so it's very small. initial drying it looked even and matte. then dark spots started to show up. i hit it with a hair dryer and they slowly disappeared. minutes later, they showed up again but in a different area or in a greater/less amount. there is no rhyme or reason. test spot on separate piece of plastic seems to have dried fine, although it was one coat, not two like on the model.

- paint: Vallejo Model Color black water-based acrylic paint

- thinner: none, minimal distilled water

- substrate: clear plastic, bare and clean

- application medium: brushed - brush was clean as a whistle prior to use

- painted in the evening two days ago

the first picture is an example of what it looks like with the type of spots in question. these will vary.

the second picture is what it looks like when warmed up with a hair dryer.

the last picture is after brushing it with distilled water (tried that for the first time tonight). you can see a bit of a different sheen in the middle - in "real life", it looks matte and very uniform - that's just the little bit if reflection of my work bench lamp.

as you can see, both of those pictures appear how it SHOULD and to me, are satisfactory. but it will go back to having the random dark areas (albeit likely to be different areas than shown in the first pic) in due time, i'm sure.

is it possible that it's taking that long for the water in the paint to leave from the initial painting two days ago? it's humid here but not that bad. i was going to just go over it with Vallejo matte varnish but i wanted to give it some more time and also, see what the manufacturer says.

thanks.
 

Edited September 24, 2015 by builthatch

[Helper Monkey]

That sorta looks like Support Induced Discoloration.  Basically it is when are impurities in or on the support that the pigment to discolor discoloration. I have seen this happen to a lot on people who paint acrylics on wood supports. It can happen with plastic too. In the art community it is common to seal the support with something before painting. A popular item is Golden's GAC 100. I have heard of people using Gamblin's PVA sizing, even thinned down PVA glue or shellac (only for use on oil based paints). Of course I am coming from an artist background here. 

I would say strip it, clean it really good with soap and water, clean it again with 90% isopropyl Alcohol and finally paint it again. If that doesn't work do it all over again but seal it before you paint. If it still does it I would say its the paint.