What if you don't have a decal sealer?

Well for so many years I can't remember, I've used this:   http://www.krylon.com/search/?search_terms=fixatif   for artwork, to keep watercolors from bleeding.  I haven't tried it for decals though.

Reid 

Future?

I've tried Krylon before and could never get the coating right - often had cracking.  Switched to Testor Decal Sealer and that worked better.

I think Future would be a no go, too hard and your decal wouldn't conform.  But I've never tried it.

Never tried the dull/gloss coat spray either, might be okay.

Paul

I've used both Glosscoat and Dullcoat with success. I have also airbrushed thinned polyurethane varnish.  They make that in a semi-matt as well as gloss and matt.  I have also used spray cans of clear acrylic spray, but found, at least in the spray can brand I used, it came out in a very thick, unrealistic coat.  Would look okay on custom cars, maybe, but not other things.