Question about Craft Paints....

Hi all.

Here is a link to an older thread I did on decorating my coming baby's nursery.

http://davesgarden.com/forums/t/593726/#new

I am planning to do a simple mural on one wall of the room but due to the number of paints I would use I was going to buy the cheap crafters paints (you know, $1 a bottle ones like Delta, etc). The question is, how can I make the mural part semi-gloss to seal and preserve it as the paint on the wall will be semi-gloss?

HD said they have a clear seal coat but it comes in a spray can! If the sheen is slightly different on the mural vs. the wall I want it to only be on the mural so a spray won't work. I don't mind brushing a clear coat over the whole mural. Is there a clear acrylic sealer you can buy? And also, would it discolor any white craft paints? I used crystal clear on some wood painted projects one Christmas and was disappointed to see that after several months it had discolored the white paint to yellow.

Let me know if you have any solutions for me. Thanks!