OK, so masking with nothing but tape as in the Gundam article is all well and good but sometimes just masking tape isn't enough. The edge could be too irregular or the angles impractical...

On these pieces (from a Bandai HG Aestivalis kit) the edges of the areas to be painted are awkward - it's not a straight line or an easily-tapeable one, so we have to resort to more drastic measures.

First, as can be seen above, we paint in from the edge a little with a brush in the final colour. The idea here is that we can guarantee a good edge with a brush (use slightly thin paint so make it easier to be neat and reduce brush texture in the paint, don't worry too much about coverage as we will be airbrushing later and we can touch it up if need be) where masking tape wouldn't necessarily be so clean.

If you have access to some masking fluid, it can be helpful - masking fluid is a bit like quick-setting PVA glue, it doesn't stick to the paint, rubs off easily after it's dried but won't let the paint through. Be careful to rinse the brush you're using to apply it with frequently enough, though, or it will dry in the bristles and destroy the brush. Use an old brush anyway, it's not nie brush-friendly stuff at all. If you don't have any, don't worry too much.

Next, tape masking tape around the piece to make sure we don't spray anywhere we shouldn't and airbrush the colour on.
If you're not using masking fluid then you'll want to tape over the edge so that you're masking bits of the part you want coloured as well - this is why we painted up to the edge, so that we can mask over some of the area we painted earlier and the mask edge isn't actually the same edge as the colour edge - if we don't get a clean edge it doesn't matter as both the paint underneath and the spaint sprayed over the top will be the same colour anyway.

Once the paint is dry and the masking tape/fluid removed, you should have a nice clean line against the edge of the piece. It helps, of course, if you can have some of the base colour and some of the new colour on-hand to touch up any dodgy areas with a brush afterwards.

The foot is a particularly good example of how well this technique works due to the awkward rounded corners near the back of the light grey part.

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