How to seal the ceiling
What are you looking at sealing it from?
Here's a post that might help - http://www.eiwellspring.org/saferh/SealingWallsandCeilings.htm
Hello Irene! Not sure what you are concerned about regarding your ceiling. If it is ceiling that has stains on it and you want to cover the stains you should use a primer like Kilz first and then once the primer is dry you can paint with ceiling paint. Hopefully this is the information you are looking for!
Unsure what you mean by sealing but you have several options for ceilings. You can hang sheetrock, do styrofoam ceiling tiles, metal ceiling panels, plastic ceiling panels that look like metal.
Normally a coat of paint is all you need unless it is stained. Then you need to seal bad areas with Kilz or poly before painting. If you have a popcorn ceiling, I would recommend getting it removed. Don't paint over popcorn, because it is more difficult to remove later.
Don't understand why you would put anything on to seal the ceiling. A coat of primer followed by a few coats of latex paint should be all you need.
If you need to cover a color or stain, Kilz is my favorite.
If your talking about the edges use white caulking