I spray painted my rifle a couple days ago and used the sponge technique.  For paint prep I first sprayed engine degreaser on, let it sit, then toweled it off.  Next I sprayed brake cleaner on and let that dry.  Then I painted.

A day after painting and I noticed a few places didn't dry all the way.  Right around the trigger pins and rear takedown pin, along the seam between the upper and lower, just behind the barrel nut, and the back of the receiver extension are still tacky.  I think there was probably still grease or oil near these places when I painted, preventing the paint from sticking.

Will the paint eventually cure despite this, or will I need to scrub it off and reapply?

It might be grease, or you might be putting the paint on too thick.

Try a blow dryer or heat gun (low to to medium heat). If that doesn't work you'll need to start over.
I like to use 70 to 90% alcohol from the bargain store.

What kind of paint did you you use?

I used krylon camo.  I think its grease and oil that was drawn out when I sprayed on the engine degreaser.

I had the same problem.  It didn't dry immediately and was (very) slightly tacky for a while.  Eventually it did dry.  So give it time if you have patience.

It will dry.  If you do it again, hit it with alcohol then follow with compressed air.  

Ultimately, it's a spray paint job - wear that shit out.  I didn't prep mine at all.

engine degreaser?  fail.  use rubbing alcohol or denatured alc as your prep wipe down.

THIS

Way over thought the prep process.  Rubbing alcohol, and within minutes you can paint.  Rattle can paint dries pretty damn fast on a lower(as long as your not going to heavy on the coats).  You definitely still have oil/grease where its not dry.  

If your going for the worn look, you did it right though.  Wipe away the wet paint, and youll have that worn look already

Engine degreaser is dang near diesel fuel with an emulsifier. It leaves an oily film. You should have just used break clean or acetone.

I would just rub it down with acetone or rubbing alcohol.

Done

Places where oil is seeping like trigger pins and takedown pins need extra cleaning/drying of oils.

Personaly, I have found the simple green degreaser, or purple power degreaser, and HOT water to work the best.  I use it for alua Hyde jobs (epoxy based) without issue.  Cheaper and non toxic.
Just scrub the shit out of it in your kitchen sink.