For a whole load of info re the fabled Johnson’s Klear, go to;
http://swannysmodels.com/TheCompleteFuture.html
Applying any clear coat, be it Klear, or a Humbrol mattcote/Satincote/Glosscote, evens out your finish on the model.
Glossy coats, such as Klear, have an added bonus of helping to reduce the ‘silvering’ of decals on your model.
Matt paints are rough under the microscope and hold microscopic bubbles of air. This shows up as silvering.
A glossy coat does not hold air bubbles so well and so silvering is reduced.
Klear can be applied with a good quality large soft paintbrush or through an airbrush. Apply thin, even coats and allow them to dry before applying a second coat.
If you use an airbrush, don't attempt to speed the drying process by fanning the clear, as it will not gloss properly unless allowed to airdry naturally.
You can then apply your decals to your model. When the decals have settled themselves in place and are dry, you need to seal them in. This helps make them look like a part of the paintwork and will stop them drying up and flaking off in future.
You decide what final finish you want your model to have and apply it over the Klear and decals. (Humbrol Mattcote and satincote are well recommended).
Some modellers use MicroSol and MicroSet. Some have discovered that Klear can serve the function of both of these products.
Having soaked your decal so that it can be removed from the backing paper, the decal can be slid directly onto wet Klear and manoeuvred into place. As the Klear dries, it will draw the decal to it and the surface of the model.
As polystyrene cement works by melting the plastic pieces together, it is generally felt that adding Klear to this mix of glue and plastic may form a weaker joint.
Klear can form bubbles.
This is closely related to the point above. However, even a small amount of Klear, applied with more vigour than necessary, can form tiny bubbles. These bubbles will then remain as the Klear dries and, when it's all cured, you're left with either small lumps or tiny little circles. Obviously, this spoils an otherwise flawless finish.
The solution is to carefully brush the Klear on, in nice, even strokes.
Klear can crack or craze
This is usually caused by applying the Klear on top of a relatively thick coat of paint that hasn’t dried properly. Also, applying too many coats of Klear, on top of each other, without waiting for them to dry properly, will cause the same problem.
Klear doesn't always cover evenly.
After the previous point, this seems to be the opposite. This observation came about when I was coating my DH110. I didn't want too much Klear on, (for reasons outlined above), but the Klear I was applying just didn't cover the complete surface, leaving bare patches.
I let it dry and re-applied, but I then noticed that it still wasn't covering. I paid closer attention to the areas untouched and realised that the Klear was being repelled, in the same way that water is repelled off a water-proofed surface.
I realised that the grease was my fingers may well be the reason.
Taking a tip from Tommy, I washed the model in washing up liquid, making sure it was rinsed and then properly dried.
When I then re-applied the Klear, coverage was total.
So...before applying Klear, think about giving your finished paint job a wash first.
Klear is sticky, for some time after application.
So...you've finally achieved a flawless, streak-free finish with your hairy sticks. You even managed to spot the odd stray hair deposited by said hairy stick and remove it, before it became encased in Klear, like a fly in amber.
You then carefully brush on your Klear, onto a pre-washed surface. Not too much, just enough to completely cover the model, which you then set aside.
On returning to the model an hour later, you notice various bits stuck to it; a money spider, (deceased) and a couple of very fine cat hairs. Picking off the spider corpse leaves a tiny nick, that you might be able to live with. Removing the cat hairs are well-nigh impossible, but you manage it...and leave two, fine, cat hair impressions in your now not-so-flawless finish.
So...as I said...Klear is stick for a while after application.
To keep it safe and free from airborne bits and pieces, stick your model under a large Tupperware box, or similar. This may slightly delay the curing time, but at least you won't have to sand it down and start again.
Klear darkens a finished paint job.
Stands to reason, really. If you wet anything, it gets darker. Thing is, when you apply a standard varnish, such as Mattcote, Satincote or Glosscote, it doesn't markedly affect the colour of the paint. Klear does.
This may not necessarily be a bad thing. For my H155 Olive Drab-covered 1/144 DHC Buffalo, it created exactly the right shade I was after. However, for my 3-tone RNZAF Strikemaster, it seemed to increase the contrast between the two greens and the tan-brown, (just as wetting a pebble can show off the myriad of colours contained within). Whilst I can live with the result, I'm not so sure about its authenticity. Application of Satincote reduced the contrast slightly
If colour is important to you, I strongly suggest you test an application of Klear on a piece of scrap, suitably painted, before committing to your almost-finished model.