2 possible reasons when waterslide decals dun stick to the surface:-
1) old decals...therefore the adhesive on the decals is gone or very little or something
2) soaked in water too long. reason you soak in water is to moist the "glue" and causes it to be sticky. release agent...chemical reaction kinda thingy. so if you soak too long...the remnant of glue on the decal is less. so it won't stick well. this is worse if the decal is old.

u can make it a habit on using mark setter. not a necessity if the decals are good but yes, it does make it stick better.

http://www.rlm.at/cont/thema11_e.htm

this link should cater for explanation on applying decals
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airbrush usage. the length of time is dependant on the type of compressor. diaphragm/oil-less type heats up quickly, therefore you can't keep it on too long. if not the motor will burn. for that...it'll be max 35 to 40 mins. you can continue to keep it on but my advise, nothing longer than 45 min.

those with tanks on the diaphragm type compressors...u can keep it up longer...i keep it on a regular 45 mins to 1 hr then i off it. by then, watever needs to be painted are done. for both types of compressors, make it a habit to off the compressors when u're not using...i.e. tidying up the kit, mixing paints and preppin things up.

then they r the bigger oil piston type compressors. silent and basically u can just keep it on all the time. the tank handles everything. n since there's oil in the piston...overheating is minimal if not non-existent.