Just like the title asks...I recently did a custom and the cape remained tacky even after DAYS of drying. Just wondering what I can use to seal soft rubber in the future, thanks!

It's probably more to do with the type of paint you're using than the material of the cape since you've already applied it. Depending on the type of paint it is, get the proper type of varnish. It may flake off, though, but it probably won't be tacky anymore.

Sorry, neglected to mention that little detail

I used formula p3 acrylics

I find that if you use Tempra paint to primer the rubberized part, seal it and then use acrylics over that, you get paint adherance with no tackiness.
Worked on my Black Knight custom's cape.

Ahh, good idea with the tempra. What do you prefer to use for sealing?

I find that acrylic paints are best for coating soft plastic. Use warm soapy water to get all of the factory mold agents off and then I use glass cleaner as an extra precaution to get it CLEAN.
Tamiya has an excellent rattle can primer....fine white I think its called. Let that dry for a bit and you can usually use whichever paint brand over it.
As far as sealing goes, Future floor polish. Sounds silly but its a staple on any customizers bench.

I use Testor's Dullcoat- it makes even non-custom MU figs look 100% better- by making their costumes look like fabric instead of plastic!
Just dont do Iron man or Ultron!

I'm in total agreement on Tamiya Primer- the best stuff anywhere.

But often instead of that, I spray Krylon Fusion black for plastics (available at any hardware store). Really rocks as a dark basecoat too!

Very nice! Thanks for the input people! Helping this noob customizer to get off on the right foot =)