You can certainly paint
Sailrite recommends acrylic from Dick Blick or similar. Read this post:
"Q: Hi guys, does anyone have experience painting logo's on dacron sails. What type of paints did you use or suggest and how did you apply the paints.
A:Any artists acrylic paint should work. I use Grumbacher's "Hyplar" (check my spelling). We will have 18 inch adhesive backed stencil material in the next catalog (and on the net very soon). It is available now -- use part #1013 -- it is $4.25 per yard."
Also, sailboats aren't the only places you'll find Dacron.
People who make ultralights, hang gliders, kayaks, and Radio
-Controlled model airplanes cover their wood and metal frames in Dacron, in weights similar to what we use for our mainsails. They then paint them. (Check out this replica P51 Mustang
built from wood, covered in Dacron, then painted to look authentic!)
Some recommend a good oil-based paint, while others have had luck with exterior grade latex.
Granted, RC model airplanes and hang gliders aren't exposed to the elements as much as your mainsail may be, and also, the Dacron used in a hang glider isn't furled at the end of the day. I'm not sure how well a painted main would stand up to being furled and stretched repeatedly, nor am I sure how well a painted main would hold up to being exposed to the elements for hours at a time for years on end. While your main won't be up 24/7, it will be exposed a great deal more than a RC model plane or hang glider would be. While the paint probably wouldn't hurt the fabric any, the paint itself would be tough to maintain an keep looking good. It could turn into a big eyesore of bleeding, fading, chipping, or peeling paint by year 3!
Google "painting Dacron" and you'll find a metric buttload of useful advice.... mostly on websites where people build hang gliders, ultralights, dacron-covered kayaks, and the like.
Ultraviolet Absorption of Latex Paints by Kirk Huizenga
UV barrier Latex Paint, using Latex paint as a UV barrier on aircraft, latex paints as UV barrier for ultralight aircraft fabric.
Re: Painting Dacron
Re: Painting Dacron
From "Latex Paint for Boats by Dave Carnell":
"Platt Monfort recommends for waterproofing the Dacron® skins of his Geodesic Airolite boats “...the simplest method being a good quality
exterior latex house paint.” "
Latex Paint for Boats
Painting with Latex House Paint
One of our Fly Baby restorers is painting his aircraft with latex house paint.
He's been posting his progress to the Fly Baby mailing list, and is allowing me to repost his messages to my web page:
Painting with Latex House Paint - AviationBanter
A Pietenpol builder by the name of Kirk Huizenga did a series of
spectrometry tests on several colors of latex. Turns out that there's
nothing special about black, nothing at all. Paint it with whatever
color you want, they all protect from UV equally well. His report, the
transformed data and the analysis are available (pdf & xls files):
File Library Detail Kirk's Piet Files
Painting a Fly Baby with Latex House Paint
Painting a Fly Baby With Latex Paint - Fidoe
Hope any of that helps!