Razor sharp masked edge-How?

Use good masking tape, like Tamiya or other brands aimed at scale modeling.

Seal tape with a coat of paint the color of the surface you are putting the tape over.  That is, if the color you have just painted is white, and the tape is going down over the white, spray a coat of white over the edges of the tape.  That way any paint that does leak under tape will match the underlying color.

Use coats of paint as thin as possible to avoid ridges.  Sand any ridges with very fine sandpaper (# 1200 or finer.  Use airbrush to get thin coats, not rattlecans.

DOH!   Thanks Don. You reminded me of when I used to do a coat of clear after the ape on the RC stuff to seal the edge.  Forgot about that.  I like the idea of using the color coat though.

As had been said, use a quality tape & burnish the edges to further prevent bleed.

Build up light coats on the edge so that the masking doesn't get damp. Any dampness will increase the possibility of bleed.

Mask in a clean, fluff free area as any fluff / dust on the masking edge will prevent a good seal & again increase the risk of bleed.

Don't spray toward the masked edge, it will leave a ridge / step that will have to be removed. Spray from the masked side & don't be tempted to spray from the other side to ensure that paint gets into the edge properly, as it will inevitably get into the edge just that little bit too well.

For challenging curves consider using vinyl pinstriping tape (very flexible), or Parafilm M (even more flexible).

Artists low tack Frisket can be laid over your work, the pattern marked , the Frisket removed, cut & then applied. The same can be done with overlapping strips of masking tape, but the step in the overlap needs to be burnished well.

Us 3M Scotch brand 218 tape I get good sharp clean no blead lines. ACESES5

Thanks again for the advise.

ACESES5

Us 3M Scotch brand 218 tape I get good sharp clean no blead lines. ACESES5

This is the very first line out 5,740,000 Google hits

3M Fineline Masking Tape 218 for results that demand a sharp color separation with a flawless paint line for your vehicles and other precision applications

I think I will try this stuff

3M 471 fineline tape (as used in bodyshops) is Vinyl & I belive more flexible / conformable than Scotch 218?

If I may offer another tip, that I've used, and that's Post-Its.  I use them for Luftwaffe splinter patterns, and also for masking a panel to pick it out from the surrounding panels, like the weapons bay covers on a P-47 wing.  Then I apply the color in a couple of light coats, to avoid loading too much paint on at once.

Best regards,

Brad