Plastic sheathing and masking tape was not available at the end of the painting project for whatever reason, I have no idea, but you have a Paint Brush that if not cared for will eventually dry up. Last solution that works well, is to take a sopping wet rag, wrapping the Paint Brush up like that burrito you missed wrapping up earlier. This will buy you some time until either you use the paint brush again, or you can properly clean it.
Basically what you're looking for is a good, solid foundation of existing paint that is stable and still strongly bonded to the car. Many times an original factory paint job, even after decades will meet these criteria.
Plaster. New dry plaster in good condition, which is to be finished with a paint other than water paint, should be given a coat of primer-sealer and allowed to dry thoroughly before being inspected for uniformity of appearance. Variations in gloss and color differences in the case of tinted primers indicate whether or not the whole surface has been completely sealed. If not, a second coat of primer-sealer should be applied. If only a few "suction spots" are apparent, a second coat over these areas may be sufficient.
Take off the chrome molding around both the windshield and rear window. You can pick up a special tool at your local Mustang shop or by mail order that removes these quickly without scratching paint. Of course your paint is probably not in great shape at the moment, so a large flat screw driver will also do the trick. For some classic cars it makes sense to even remove the windshield, but classic Mustangs have chrome molding that will help cover any slight overspray.
CONCLUSION. Once again go through the images and the story of the painting. Was your first impression different from what you see now? Take a 5-minute online test to explore your preferences in fine arts. In 5 minutes you will confidently say "I prefer impressionist cityscapes", or "This room calls for a color field painting", "I need to have an art nouveau nude in my bedroom" - and you will know what you are talking about!
The area covered by a gallon of whitewash depends upon the nature of the surface, but ordinarily a gallon will cover about 225 sq. ft. on wood, about 180 sq. ft. on brick, and about 270 sq. ft. on plaster. The formulas mentioned will make from 10 to 14 gallons of whitewash. If a smaller quantity is desired, the amount of each ingredient should be reduced proportionately.
Painting ceiling does not require premium paint. You can use flat $15 to $20 ceiling paint. I use Sherwin Williams PRO-MAR 200. Painting ceiling requires a lot of physical work and plenty of drop cloths. If you are not up for the challenge or have very high ceilings contacted a professional will prove to be a smart move.