Old decals won't slide off paper.

These days I always scan all my decals before trying to use them.  That way, if I have any problems, I can at least try to reproduce them with inkjet decals.   If there is no white in the decal, I can generally make it work okay.  However, decals containing white or very light colored areas can be a real pain.  Still, in absence of obtaining new ones, that is often the only option.

It might be that the decal film is adhering to the backing paper that the original decals didn't cover.  Try trimming very close to the decal image, inside the original decal film border.

Paul

That was the problem, Paul!  Thanks for the tip.

When old decals won't release from the backing paper, I let them sit a long time in warm water, maybe 1/2 hour or so.  If they still won't release, I think they are a lost cause. 

Sometimes, this can happen if the decals have suffered water damage or been exposed to high humidity (activating the adhesive).

If you soak the decals for an extended period (as mentioned above), you may lose much of the adhesive that makes them stick to the model's surface.

Normally. if this happens, no amount of setting solution will help the decals stick (with the exception of Gunze's Mr Mark Setter, which contains a mild adhesive) if there is insufficient adhesive.  

If you are able to get the decals free of the backing paper, you may wish to consider applying a very thinly diluted solution of white glue to the decal location before applying the decal to help it stick.

i had some that wouldn't slide off, i think the backing glue had dried out making the paper basically sandpaper. after soaking for a long time, i eventually got the decals off by lifting them up with tweezers, rather than sliding them off.

A warning on this to folks trying to make their own (inkjet) decals.  When sealing them with a clear overcoat, this must be done before cutting the decal close to the printed graphics.  Also, make sure you do not get the clearcoat on the back of the decal.  Getting clearcoat on either the edge or the back of the decal inhibits water from soaking the decal.  So leave enough of a border that you can trim the decal before soaking and that trimming will get rid of sealed edges.  And coating the whole sheet instead of individual graphics makes it easier to keep the sealant off the back.