Thank you Gregory, RagIII and everybody,
I can show here some more work done since last update:
Printed the home made decals:
First sheet is a "clear" one

and is a white sheet

I'll made my first try with the big flag painted on the fuselage printed on the clear sheet.
On my previous SVA 5 I used the white one and had to cut each of the pieces... really tiresome and I discovered I have a low level of cutting capabilities
:-)
The long "5" row outlined in green is a try to get the white "5" on the fin and tailplanes, I wanted to cut them with a lancet.
Then I found these markers with white ink in a stationery shop:

Here are their tips:

On top the Pilot, below the Uniball Signo (which is nothing else than a gel ink Pen)
So I decided to try the, and I cut a clear "scotch" removable tape and paint the 5 with those markers.
Here are the terible results of some tries made on a black Plexiglas piece:

The arrow point at a try done with the Pilot marker on a piece of clear decal sheet coated with transparent opaque finish, too liquid and uncontrollable...

The ink gel pen makes a thick mark on the surface where it writes, so that it appears as a relief mark so it can't work in my case.
So in the end I decided to let everything down and go by freehand: I fixed a printed paper sheet on the piece and tried to slightly carve the angle points with this iron point.

Then I joined the points and drew the 5 myself: this is an intermediate stage, I proceeded scratchimg off the green paint and letting the white primer coating appear from below

Then I touched it up paintting over with a off white colour.
This is the final result:

I am pretty satisfied with it
Here you can see the current state of the lower wings: painted and weathered

This is the work done on the upper wing:

Camouflage and preshading appearing on the roundels' spot

Roundels in place

the roundels were shiny and glossy, the colours were too bright, so I coated them with matt varnish and washed the surface with a _very_ diluted medium gray.

Detail of the gravity tank

Lower surface of the top wing.
That's all for today.
Thanks for stopping by!.
Antonio
PS what would you have done to paint the 5 on the tail surfaces?

?