Hi,
What's the best way to clean your plastic of resin model/figure before priming ? Just water and soap or are there better ways?

or water + dish washing liquid
Some older east european kits had some mold release oil on the sprues which sometimes needed solvents like white spirits to remove. Modern kits don't usually have this issue, the only grease on them parts will come from your fingers and that grease can be removed with soap or dish washing liquid.

Try the soft methods first and if that doesn't work it could be time to start using white spirit or turpentine.
/ Robin

I used to use a drop or two of dishwashing detergent in a tin pan of warm water for "washing" my styrene kits, but I don't anymore for the reasons Robin stated above. However, I always wash resin kits, because the mold release agent is often rather heavy for resin. Having said that, I was painting a Roden Nieuport 28 kit recently (I know, not Armor, but it's still plastic!), and when I got to one section of the wing, I noticed my highly thinned Tamiya paint was being repelled by something on the finish, leaving small circles of bare plastic where the paint wouldn't adhere over a two inch long area. I quickly used a thinner soaked towel to wipe the paint off, and continued to paint, that seemed to work. I think with today's more "modern" kits, you don't have to worry much about mold release agents.

Now for two "horror" stories. #1: I was building Revell's 1/96th HMS Bounty a few years back, and ran across a "the mold release agent from hell" on the masts. I could not remove it (this is when I was still washing my kits). I tried everything, but even when I laid down a coat of primer, it would just dry and peel off. I resorted to a short soak with mineral spirits, but I still couldn't get any paint to stick. When it did stick, it would "split" down the length of the mask and flake off. I was using enamels, and even resorted to mixing them with lacquer thinner. I finally resorted to using steel wool to "sand" the masts, followed by a white colored spray lacquer designed for metal. That did the trick. ---Story #2: a few years back I was building a 1963 issue of a 1/48 WWI Breuget 14 from Aurora. Those familiar with these old kits know they have all the "insignia" molded into the plastic. I did extensive re-working of the kit, including sanding and rebuilding the wings. When it came to paint, nothing, not even lacquer paint, would stick to the upper wing-- the rest of the kit was fine, but the upper wing repelled paint like "water off a ducks back", even after the wing had been entirely sanded down! I was able to get several coats of paint on, only to find it would peel right off when I was masking or handling the model. I finally found some heat resistant automotive engine primer that thinned with acetone that stuck, allowing me to get a coat of primer paint on. So, washing might be important, but it won't always guarantee a paint adhering surface. Fortunately, I think those old kits are the ones you need to be careful about-- not the newer kits.
VR, Russ

Great, as for now I'll go for "a drop of dish-washing soap". When you are painting your woodwork one uses ammonia. Would a drop of ammonia in a bowl of water work? I tried it today with some old plastic spareparts and they didn't damage or so.

Hans,
I'm pretty sure a drop or two of Ammonia won't hurt, especially since most household Ammonia is already diluted to 10% or less, but it depends on the paint you are using. Pure industrial Ammonia is another issue-- and can be quite toxic as well as caustic, not to mention downright stinky. I'd be cautious about using ammonia if you are using Acrylic paints, as they can react badly with ammonia, and ammonia is what's used to remove Future/Pledge/Kleer. Never use Ammonia with Latex based paint products, since Ammonia is already a key ingredient in some of them, and addition of ammonia will only weaken the paint's chemical bond. If you are using enamels or lacquers, you shouldn't have any problems. If you are really having a problem with a particular mold release agent, you might try soaking in Windex (an Ammonia based glass cleaner here in the states) followed by a rinse in warm water. Windex is a pre-mixed solution with Ammonia as the primary ingredient. I have used foam style kitchen oven cleaner as well for cleaning plastic, but I'd caution you to be careful, wear gloves, and don't allow it to soak in the foam for more than a few minutes, and never soak chrome parts, as oven cleaner will strip the "Chrome" (which is really just a thin coating over a layer of clear enamel). Oven cleaner is used frequently for stripping paint off plastic, but I'd test a piece of sprue first. Be aware that not all plastics are "created equal", thier formulations can vary between manufacturers, which is why some are brittle, hard, or soft depending on the kit. For instance, I've found Roden uses harder plastics than RoG, Airfix is usually softer than Tamiya, etc.
VR, Russ