Martin,
The tissue paper can be used to help cover up tiny voids, like the grain of the wood, or micro cavitys in body filler. It's a good method to use if the wood underneath is in bad condition, but if your going to be using 3-5 coats of Hi-lift primer (really bulky primer with Talc added to it ) there is usually little need unless you've made big mistakes.
From our discussions on your builds, in your personal situation, I would recommend glassing the ply wood, instead of using tissue pape as your going to need LOTS of practise with epoxy when it comes to the armidaler. Similar if not the same method as FLJ uses for tissue:
1) use 25gsm finishing glass, drape it over the thing to be covered
2) pour on a tiny amount of resin
3) Spread with a playing card until it'll all taken up and you've got a lovely smooth surface
4) Once dry, break out the Hi-Lift.
For really tight curves, like if you have a strake that is jutting from a hull, as it can be a pain, as the covering is dragged off the "sticky out bit", and you loose detail and it looks like you have bubbles under the tissue paper. Glass is less affected, but it still happens, so you just leave these bits alone if your covering. Or, you Vac-bag the piece your working on, and everything sticks down properly.
There is, however, a much quicker way which gives as good, if not better, finish. Again, like FLJ, this is only for Pre paint work, although it's good for using on your first 2-3 layers of primer. I also takes what ever your doing down to a completely mirror finish, so if something is meant to actually BE wooden on the model, it's not for these sections.
All you do is get a get a few packs of generic stanley knife blades, which go for something like 60p for 5, and, holding the blade between finger and thumb,, perdendicular to the bit you want to flatten, start to drag and scrape the blade forwards and backwards. Start with the grain, and then when your down to close to a mirror finish, you can go accross the grain.
You'll be suprised how rough you can actually be with this method, as you can REALLY attack the piece with no problems at all. To give you an idea of it's effectiveness, i did the entire top deck of a 700mm boat in maybe 15 minutes probably less. This includes working my way through some fairly rough body filler and a cyano spill. The ply is now so smooth that I had to move it into the shade, as it dazzling when it was sitting in the sun reflecting it all over the front room.
Works perfectly on curves etc as well, as is still quicker than blocking off with sand paper, you just have to do a bit at a time. For small area's, just cut down a few stanley blades into 1/2 and 1/3 size sections.
Again, I'll say that you shouldn't do this if you you actually want to KEEP the grain. I forgot, and used this method on an interior section, and how I'll have to paint a grain on by hand.
Steve