Quoted Text
...the paste I used must have had alcohol in it.
Marc, hmm, alcohol seems unlikely in toothpaste, possibly the soapiness did it? Maybe your paint wasn't fully cured, or maybe the plastic hadn't been degreased prior to painting? Did you prime it first before the base colour?
I must say that is a
lot of toothpaste you've used! Almost looks like you may as well have squeezed it straight from the tube!
Here is one of those Jan refers to (MBN Campaign):
It was primed, then basecoated with Tamiya Dark Yellow, then the first application of toothpaste:
Then some Tamiya brown was added. I recall the paint was a bit too thinned, and I applied a bit too heavily, so that the toothpaste started to spread a little on the nose, but not too bad:
Then the next layer of toothpaste over the brown:
Then the Tamiya green:

When everything was washed off:
Then added the contrasting dots with a toothpick - more tooth-related tools...

Then some filtering with oils and Liquin:
I don't think the toothpaste matters much - on this one it was Sensodyne white I think, on the WWI Mark I tank it was Colgate minty blue gel (with stars, which I picked out). I certainly washed the model, then primed, then allowed the successive colours of acrylic paint to dry overnight between coats. Toothpaste was applied with a small brush, probably about one teeth-cleaning session-sized blob only. I never had any issues with the paint being damaged by the paste going on top of it. Certainly I used far less toothpaste - just a thin layer is enough to act as a mask, and it seems to me that the thinner the masking, the harder the edge you will achieve; when using blue tac etc. you can end up with a semi-hard edge unless you spray in towards the point where it attaches to the model (sometimes that is what you want of course).
Hope that is of some help if you decide to go with it again. As well as on the Mark I, I also used it on the Morris Quad in the MBN campaign, again with no issues. Your detailing on this model is fantastic by the way. I have the Airfix KT which I need to finish at some point and will come back to this thread, most especially with regards to the tools etc.