In my post of 30 November last I mentioned this product. I have now got around to buying a couple to try, and first of all let me say I was a bit naive about them adding weight. The etches are so thin that the added weight factor is absolutely minimal. (Obviously this means that the sides are kept as near scale thickness as possible.)
Really they are intended to put in new kits not retro fits and it would be easier to use them that way. (Not easy, just easier.) The sides and ends in particular are ultra thin. Thinner than paper. There's a lot of bolts to press out and unless you're careful the brass distorts to hell. A really delicate touch is needed - more delicate than mine, frankly. Same goes for cutting them off the frets, the sides and ends are practically individual planks held together by ironwork and it's very easy to distort them. You have to be really, really gentle. Also on the fret are capping pieces, wagon label holders and oddments. You can even detail the frames beneath the wagon.
As I discovered the best way forward would be to chemically blacken the etches before you start. After much fiddling I got the sides end and bottom inside my wagon (and not looking too hideously wrong), and then sprayed with primer which promptly hid most of the fine detail. Chemical blackening followed by thin coats of paint is the answer to this, I think. Even though my first effort is rather bodged - I haven't finished the painting yet, and some bits of coal dust strategically placed should hide the worst of the errors - the wagon interior looks a whole lot better than as bare plastic.
I hope this summary doesn't sound too negative as it's an excellent product and the instructions, in particular, are of exemplary quality. I hope to buy more, but whether I have the patience to retro fit them to all my Slaters wagons is another matter. They will certainly be used in all future new builds though!