Paul,
Microscale's liquid Decal Film is a godsend in a bottle.
I guess that the Hasegawa decals you are using are probably quite old. I've never had Hasegawa decals perform the 'shattering' trick on me, only older Roodecals (rather catastrophically).
Don't bother clogging up your airbrush. Get a flat broad brush, load it up generously with the liquid and use broad single but overlapping strokes over the entire decal sheet or only over those images you require for decalling the model. Allow to dry. I think this liquid has a solvent of sorts as it evaporates reasonably quickly. Don't be too worried if it looks like you've put down quite a lot; when dry it remains as a thin film. DO NOT repeatedly brush over the same location. This is not fence painting (which I am doing at the moment actually). A second coat might be useful if you are a 'worry wart', however, it will make the resulting decal thicker, which may not be what you want/need depending upon what your requirements/constraints are. Practice with a spare decal to get some confidence.
This is great for sealing over inkjet printed images or photocopied images/text on decal paper. Again, only SINGLE strokes with slight overlapping. Failure to adhere to these guidelines will result in a higher than otherwise probability that you will make the printed/drawing image run/smear. No running back to Uncle Nic to complain that what he said was crap, ok ?
Years ago I used this stuff to create decals. I did an ink drawing of a caricature and some text legends and over coated with this product once the ink had dried. I was able to successfully apply the decals in the usual way. A winner !
Oh, I washed out my brushes with an alcohol-type solvent after brushing this stuff. I expect a Windex type product would be acceptable also. Maybe a lacquer thinner also ? The proof to me was that the brush was clean and supple and able to be used again.
Good luck m8