I find decals WILL stick to doped surface, but you should at LEAST 2 coats maybe 3 before decals. The paper really SOAKS up that first coat. The second coat is a MINIMUM I would have before I apply decals. The paper will retain a nice "SHINE". Oherwise the paper is still too pourous and that is why the decal can NOT hang on. The paper needs to be completely sealed.
I find it harder to get decals to stick on the painted surface because it needs to be SMOOTH, preferably GLOSSY.
ALSO... you said 50/50... with 1 coat that is not much. I know the dope says 50/50... That's a guide... Mine is more like 2/3 dope 1/3 retarder.
SO...
1. Dope the paper until it has a nice shine, AT LEAST 2 coats, 3 would not be a bad idea.
2. Larger Decals May need to be cut, Guillow Decals are thick and do NOT mold well to Curved Surfaces. I had to make a slit in the "Meatball" for my Zero
3. They may not stick to plastic, but they are durable and will hold together... get some WHITE glue down and get that Decal ON!
Thats my 3 cents worth... Mitch... nothing is as easy as you may think... Good Luck
UPDATE: My expeience is for decals on doped paper... ALSO... If you want to paint, the same thing goes... the paper should be sealed before the paint.