Enamel and Acrylic on the same model?

Should be no problem to go over an acrylic with an enamel Paul...I do it all the time without incident. Be sure the acrylic is completely dry.   More apt to cause a problem, would be putting an acrylic over an enamel.  The main problem being adhesion.  When applying acrylic over enamel paint the surface should be thoroughly dry and lightly sanded with fine paper to give it some "tooth" for the paint to grab on to (although, most modern acrylic, hobby grade paints stick very well).  If you plan to paint the majority of a model with acrylic, I would recomend a primer coat, a thorough light sanding and then the finish coat of acrylic.  After which, you would carry on detail painting with either enamel or acrylic. Hope this helps!  Good luck with the project and above all have fun with it!!

P.S.  Paul...You're going to love the simple cleanup that comes with useing acrylic paint!!

Static. 

There is a difference between "dry" and "fully cured," and it is particularly critical with acrylics. Acrylics dry rapidly, but cure slowly. Wait a minimum of 24 hours before overcoating or masking the acrylic.

A primer is not necessary under acrylic paint except to find surface defects or as an undercoat to a more transparent color like white, yellow, etc. (That is also true of enamel whites, etc.) What is necessary is a clean plastic surface. Clean the surface with a surfactant cleaner like Formula 409, Windex, or a Simple Green solution (the latter requires rinsing.) As a minimum, wipe the surface with 90% isopropyl or denatured alcohol before painting. Do not use common rubbing alcohol of 70% concentration. It may leave a residue.

I mix the two all the time as base camoflage colors, and it's a requirement (almost) to overcoat acrylics with clear enamels or vise-versa for weathering... Like the PaintMaster says though, cured is the key, rather than dry if you have to mask (at least with tape)...  You can use low-tack masks (like Post-It notes) or wet-paper masks if you don't want to wait for the paint to cure though...   Simply cut or tear the mask (I recommend newspaper or brown paper towels), soak it in water, then apply it to the model.  Then blot it with a kleenex to remove ALL the water drops from the area, and shoot the color...

A more expensive alternative is to use Frisket Film—a little easier to control than the wet mask, IMHO. But don't neglect the wet paper method, even on fully cured paint. Because the paper in paper towels (the non-fancy types that Hans recommends) will readily dry in a shape conforming to a curved surface and contain no adhesive, they can be better than any other mask. Dry them with a hair dryer or just a fan blowing over them.

And whatever you do, don't leave masks on longer than you have to, regardless of paint system.

I use "cling film" food wrap for masking quite often.

Acrylic, and enamel are both soft paints, and they are compatible with each other, so there's no worry about ruining your model if you want to use one, or the other.

Triarius

A more expensive alternative is to use Frisket Film—a little easier to control than the wet mask, IMHO. But don't neglect the wet paper method, even on fully cured paint. Because the paper in paper towels (the non-fancy types that Hans recommends) will readily dry in a shape conforming to a curved surface and contain no adhesive, they can be better than any other mask. Dry them with a hair dryer or just a fan blowing over them.

And whatever you do, don't leave masks on longer than you have to, regardless of paint system.

I use frisket paper too... Especially for making national insignias on armor & soft-skins... I also prefer to make the hinomaru for Japanese aircraft and Soviet red stars from frisket masks...