Printing my own decals - how to seal them?

You have opened the biggest can of worms in inkjet decal making.  The trouble is some inks are more resistant to sealing coatings than others.  You need to find the right kind of ink and printer.  I had an old Epson I tried to use for decals, and everything I tried to use dissolved the ink!

Finally bought a cheap HP printer that I only use for making decals (it was on sale for forty bucks). I bought it with the understanding that I could not make decals with it I could take it back.  Works fine. I have tried Testors glosscoat and dullcoat, polyurethane varnish, and clear acrylic spray.  Haven't tried future but I bet it would work- it is very similar to the clear acrylic spray I used.  I happened to keep those coatings on hand anyway.

Since then, I bought a new Epson printer for general use. It uses a different ink series and I find I can use it to make decals, but since the HP inks are cheaper I still use that Deskjet 2512 for all my decal work.

Microscale also makes a liquid decal film that can be brushed on.

Hmm, thanks Don.  I have a HP Photosmart B109.  I've ordered a single sheet of clear decal paper so I'll give it a go.  Good to know that you think Future should work.

Well I can confirm Future (at least the bottle I've got) does indeed seal decals that have been printed onto a sheet of inkjet clear decal paper.

I had to put 2 or 3 coats on with my airbrush, can't think of another way of doing it because a brush would smudge the ink and dipping them would seal the whole thing so I can't slide the decal off the backing...

Although I had the perennial problem of getting the decal to lie down properly on a curved surface (shoulder pad of a Games Workshop Space Marine Terminator).  

Probably should have cut a bit neater around the decal and bought some microsol to use on it - but would the micorosol get through the multiple coats of Future?

Yes, it is another problem with inkjet decals that the seal coat can stiffen them and make them harder to conform to curved surfaces.  Do not put on too much of a seal coat.  Correct amount is learned through practice and test strips.  Too much seal coat also causes the decal to curl upward, making it even harder to conform to convex surfaces.