I started on a custom of Leader Prime last year when he came out and the first thing I did was paint the gray, silver with Testors' Enamel Silver. I've put off finishing it until now and it's still a little sticky when I hold it, after over a year.

What can I do to fix that? Would a gloss clear coat fix it?

Also could you tell me about clear coats because I've never used one before?

 

This is something that a lot of people run into when doing customs. The reason that it happens is that enamel paint works by a process of what's called "solvent release". The same way that acrylic paint is mixed with water, enamels are mixed with solvents. They are supposed to evaporate and that's what lets the paint set. What happens to some plastics is that they solvents sink in and get trapped, leaving the paint sticky. There's really not much that you can do except to clean it off with mineral spirits and start over with acrylics.
I've found that in general, hard plastics are safe for enamels but soft plastics never react well. The soft plastic soaks up the solvents and never set.
I hope that helps, even if it might not be the answer you wanted.

 

Glad this question has been asked. Something I never thought of before cause I've always painted plastic model cars with enamel and never had any issues.
I'm considering doing a custom TF soon and was wondering about their types of plastics. If I do a custom soon, it's probably going to be ROTF Sideways. I love the Audi R8 model and I wanted to do him in white and have several of his robot parts chromed.
I still paint via rattle-cans so any tips would be greatly appreciated.
Great question and thank's for asking!

 

There are some spray paints on the market now that are specifically for plastics. Krylon fusion and Rustoeum's Plastics line are good. They both make primers for plastics too. Once that's on you can go back in with acrylics or more spray paint. The problem with the soft plastic parts still goes though, so be sure to mask them from the spray paint.

 

OK, I knew the part about soft plastics, so I didn't paint the smoke stacks. But the hard plastic IS what is sticky. It's the plastic that's gray on Leader Prime. (i.e. - feet, legs, and gas tanks) Why is that still sticky?

 

Looks like you just got lucky. Bad paint batch maybe. Might be a quirk of whatever plastic they used. There's really nothing to be done except clean it off and start again. You can't put anything over top of it or it will be like trying to paint a fish that you pulled out of the Gulf of Mexico.
The best metallics are enamels, so you might be able to get away with using it if you give it a good primer coat first.

 

  It's not that sticky. It's fine if I just hold it to move it, but if I hold it for a while it starts to come off on my fingers. I should probably rephrase it to: It's a little tacky. Nothing sticks to it except my fingers when I hold it for a while, like my body heat is warming it up and making it come off.