The small HUMBROL enamel paints are nice to use.. they have a perfect consistency to flow off a brush very nicely. I do a lot of the free hand black lines with these.
You can always just "spot "seal the touch ups.
Either just dab a small amount of coating on with a brush.. OR.... cut a hole in a piece of paper the size of the touch up - hold just above the PF and give a VERY short and sharp spray with a can of polyurathane... you need to do a tiny amount holding the can about 6 inches above the paper. The coating will spray through the hole and disperse outwards leaving no visible lines and will blend in nicely.. allow to dry completely and cut back with T cut or similar - buffs it right into the PF..
Another disadvantage of water based acrylic is they tend to shrink when drying.... no matter what paint you use ensure PF spot is grease free by wiping with metho first, or ANY paint will just lift
