We found 4 Reddit comments about Maskol (ac5217). Here are the top ones, ranked by their Reddit score.
This stuff? Humbrol Paint Maskol 28ml Bottle https://www.amazon.com.au/dp/B006ZY3KZW/ref=cm_sw_r_cp_apa_i_n4czCbKN656Y6
Not super cheap but this will do the trick. You can also use poster tack (YMV with different brands). Maybe spray a coat of matte varnish on before applying tape/putty just to be safe.
Masking fluid could work too, but it has some caveats. You need to paint it on pretty thick, and don't let it sit for too long or it will be hard to get it all off. Its better for paint chips and small things, I don't recommend it for hoards of tyranids.
Masking is one of the drawbacks of airbrushing. You might be better off using a brush for the red, then carefully airbrushing highlights with just some quick masking. Then go back and touch up the blue with a brush if needed.
Yep, OP do yourself a favour and buy some of this stuff to stop overspray:
Tamiya Masking tape in various widths
Maskol Liquid Mask