Cleaning the model with thinner.

Surface prep is what separates the killer finishes from the meh finishes, but using thinner to do it isn't a good idea. Enamel thinner will not harm the plastic but lacquer thinner will for sure. Acrylic thinner is going to be completely useless.

If you're using sturdy paint it really isn't necessary from a durability standpoint. But try not doing surface prep and then mask over some Model Master Acryl. You'll have a bad day I promise.

Washing sprues before building is the dumbest thing I think I ever hear of modelers doing too. I see no point because ultimately through the coarse of the build you're just adding back skin oils, sanding dust, and who knows what else to the surface.

I use 90% alcohol and gently wipe down the plastic parts before painting. When the main assembly is ready for paint I brush the sanding residue out of panel lines, use a tack cloth for any dust etc, and then wipe the model with alcohol. I takes seconds and improves the surface to take paint like you wouldn't believe.

The only items I wash in Dawn dish washing detergent is Resin. I never wash a model and if it needs cleaning, plain old rubbing alcohol and a small cotton wad will do the job. The thinner may loosen, soften or dissolve any putty or filler you applied. Amazing some of the methods people use.

This sounds like a variation of the old "tack rag" technique for painting anything.  A tack rag was wet with a very dilute solution of varnish and thinner, and was quickly wiped lightly over a surface to remove dust.  A wet rag or paper towel also works, though not quite as well.

I have indeed used a paper towel wet slightly with turpentine not only on the bare plastic, but before finish coats.  The secret is that the towel only has a very slight amount, and you do it quickly and lightly. I use enamel paints, and I have never hurt anything on a previous finish coat. But, again, the rag or paper towel should NOT be dripping wet, only very slightly damp.  If you rub plastic with a dry cloth you create a static charge that will attract the dust right back. So the dampening helps prevent water build up, and a tacky hydrocarbon will attract the dust better, though again just preventing the static charge is a big help even without the tackiness.

Save your money on those plastic prep solvents. I usually soak the sprues in dishwashing detergent or laundry detergent for a few minutes then rinse well before drying them. Besides, a bottle of detergent is cheaper and lasts far longer than the plastic prep stuff.

No, definitely not needed.I have not even washed any of my recent models.Only resin parts or older kits that feel noticeably greasy.

Warm soap and water, alcohol, or Testor's Plastic Prep are all things you can use to wipe down your plastic parts before paint. It helps remove contaminants such as oils and fingerprints, and the Alcohol or Testor's Prep dries pretty quickly so you can paint soon after. I use Testor's Plastic Prep and definitely recommend it. It is safe on plastic and even dried paint.

I've never heard of/seen anyone clean an entire model with thinner prior to painting!

What kind of "thinner" are you talking about? A thinner for use with enamels or lacquers could very well craze, melt, damage the plastic. I would also be concerned about surface residue affecting a coat of primer.

Acrylic "thinners" usually contain water, alcohol, ammonia. A huge waste of money to not do much of anything.

I could see using regular ole rubbing alcohol or soap and water to clean a model.

No, no, no!  Not recommended!  It will likely mess up any primer or paint.  If you feel your model must be washed, I think plain old dishwashing soap and water will do the best.  Let it air dry for a day, and then have at it with the paint.

Firstly ---- what sort of thinner's are you talking about, Acrylic or Enamel thinners. Most Acrylic thinners are based on water, so they should not harm the polystyrene.

Now the Enamel thinners are another totally different beast altogether, these "solvent type thinners" can and will utterly ruin the polystyrene model, if you run out of glue for the model then thinners can be used. So before you decide to WRECK your model, please use the rag that is soaked in Enamel thinners, on a bit of spare sprue. If it starts to melt the sprue, then under NO circumstances go near the model. All Enamel thinners are made with different ingredients, some might be OK for polystyrene, but the vast majority will melt the polystyrene. Now some on here will say just "go for it", well, if you do "go for it", then please test it out first.