I want to stiffen cotton sails on a model ship. I am thinking of blowing air against them to make them billow and then spray them with hair spray to hold the billow. Would spray starch be better.

I'd consider a clear satin lacquer or enamel Starch will be effected by humidity and hair spray was never really meant to last very long. I assume this is on a model boat that you intend to have for a length of time