You will need
A pair of rubber gloves. If you have ever got a chemical burn in a hobby cut on your finger before, you will understand. Plus, the solution will sting normal skin and is an irritant, so protect your hands. You need them for painting and rolling dice.
A nail brush or similar short stiff bristle brushes. A toothbrush is useful for regular infantry size models and awkward recesses but if you are stripping down a large vehicle I use a larger brush.
A big water proof container, preferably with a lid. Those plastic stacking household boxes used for storage are ideal for this! High sides are also good - you can scrub the model without flicking gunky mess everywhere.
A good paint stripping solution. I personally use Fairy Power Spray. This stuff is fantastic at stripping paint from plastic - without damaging the plastic of course, and it costs a couple of quid a bottle, so it is fairly cheap stuff too and won't eat into your hobby budget much. (Or pinch your mums/girlfriends/wifes bottle in an emergency lol)
How to do it
Step 1: Put your gloves on. Shake your spray really well - it reacts better when mixed inside the spray bottle first! Coat the model you want to strip down in Power Spray (Simple Green in the USA). Really drench it, the gel from the spray sticks and expands in foamy gel all over the model. when it looks like a slimy bubbly mess put it to stand in the container for an hour.
Step 2. Get scrubbing! Take the model and start to brush the surface rigorously. This will break down the first layers of paint on the surface. At this point you may already be able to get the model back to a workable level if it wasn't too clogged in paint! However, if it has multiple layers of old paint you now rinse the model off completely and let it stand for a few minutes to dry off.
Step 3. Simply Repeat step 1. Fresh Power Spray all over the dry model again. It will start eating into the already weakening under layers of paint better if the model is dry again before application: water on the surface will dilute the gel/foams strength a little.
Step 4. Allow it again to sit a while and let the chemicals work the surfaces and recesses once more.
Step 5. Again, scrub the model rigorously and remove all the gel, gunk and slime from the surfaces. Rinse it thoroughly with clean running warm water. Keep scrubbing it under the tap - you want to remove any possible spray/dissolved paint. Use the toothbrush for the hard to reach spots, or use a hobby sculpy tool (not a sharp object or you will scar the plastic) to dig out the claggy mess in the deep corners.
At this point you should have a pretty clean model. If it is still not stripped down to your liking you need to go one stage further...
A soak bath will also remove a lot of undercoat spray that bonds with the plastic, but it will also dissolve the glue sometimes. Be prepared to rebuild the model kit if you soak it.
You can make a soaking bath and leave the model in there submerged for days if required. After the initial paint stripping you may still wish to remove the very first bonded undercoat on plastics. Simply add more water to the tub, Dettol or powerspray to the water, Spray the model again, leave it to stand for an hour then drop it in. Each day remove the model, scrub it clean, repeat and put back in the soak solution until you are satisfied. I have yet to find any messed up Orky paint job this does not remove!!!
Just be careful when using all these chemical solutions, wear gloves and don't flick it towards you when scrubbing, or rub your eyes obviously.
So now a before and after shot!
That is what a Stormraven Gunship looks like after being put through the basic stripping stages - 3 layers of paint were removed down to the undercoat. In the tub is also 2 Rhino APC's, A dreadnought Drop Pod and a Storm Talon I recently got in a hobby swap with a local friend and fellow gamer. This was after a day in the soak bath. the kits came apart beautifully in stages each day in the soak.
Aaaand I did a basic airbrushed paintjob on the Gunship for the lad as he has trouble getting even, brush mark free layers on his larger models. I also magnetised the weapon systems so if he wants to swap n change for missions its easy to do. I left off any chapter markings etc so he can decide where he wants them himself.
So, there ye go. Pretty simple to do, just takes time and a little effort and of course know how.
And now you know how!
Hope this helps some fellow hobbyists restore old kits and saves you a fortune in new models from the process.
Moar Stuff Soon!