Originally Posted by
Lieste
It is uncured resin, where UV light couldn't get to it, due to the coverage of supports and hull, and where alcohol baths didn't have the agitation to disturb partially cooked resin.
You can either wash it away with more alcohol, or cure it by leaving the soft part exposed to daylight, and avoid touching it to leave finger prints if in an area which can show.
There is no mould release agent, which is why plastic parts are usually greasy and don't take paint.
Some FDM filaments (e.g. Nylon) self lubricate during printing and in their life, so cannot be painted effectively, but PLA is quite easy to paint.