Yes, Silly Putty, that lump of clay like thing that you played with as a kid. Turns out it’s a great tool for doing masking, especially for camo. I have heard a few times that people complained about it leaving a residual, but I have never experienced it. Of course, I use only acrylic paint, so perhaps it’s a reaction with enamal paint? I have left it on for days with no issue – just make sure you let the first coat of paint fully try first. Here’s a picture of me using it for Tamiya’s JSDF main battle tank over Tamiya acrylic paint.
You can also roll up the edges so that it stands proud of the model if you wish to create a soft demarcation line. You can also create a ‘roll’ of the putty as it easily bends to any shape you wish, and then cover the rest with tap, as I did with this p-40 nose.
A really cheap and useful tool. Makes a blob for inserting small pieces for painting with an air brush also.