This tip describes one method to mask off and paint fin tips.

First, paint the airframe as desired, including at least part of the fin root edge along the body tube.

In this example, the entire rocket, including the fins, was painted red as a base coat.

Once that has dried sufficiently to allow handling, place a plastic bag over the fin can.

Push the fins through the wall of the plastic bag, maintaining a relatively tight seal around the fins where they protrude through the plastic bag.

Use masking tape to seal the edges of the bag and define the edge where the fin color will begin. This could be down near the root edge, or out towards the fin tip.

Spray additional base color along the tape edges. This is an important step, as it seals the edges of the masking tape, and prevents the fin color from bleeding under the tape.

As soon as is allowable by the particular paint, spray the fin color over the fin can.

Once the fin color has dried sufficiently to withstand the tape removal, peel off the tape and remove the bag.