If I can, I always use Archer dry transfers on all my armour as they are so easy and I don't have to mess about with gloss coat, matt coats, fixing and softening solutions to try and make decals look like they have been painted on.
Archer dry transfers will go on any painted or unpainted surface really easily.
Here is my method, explained with the aid of my recent Meng Panther A build.
You have the rough zimmeritt coat to contend with, but it is no problem at all as the ink used is a flexible ink, designed to stick and flex so any surface has the exact process.
Step 1 I put a thin bit of masking tape to mark where the top of the numbers will go.
I cut out each number giving me enough space to fix into place with some more masking tape. This prevents the transfer from moving.
I use a modified cocktail stick, making one end a flat edge and go over the transfer in it's entirety two or three time using the pressure I would normally write with.
You can see in the photo that the transfer looks different that it does in the picture above, as it has gone on to the paint.
I carefully peel off the transfer paper checking as I do that the transfer has fully gone on to the zimmeritt. If some remains, because the paper is fixed in place I can put the paper back and go over that bit and make sure that it goes on to the surface. If this happens you can't tell at all!
I do the same with each number making sure that the spacing is equal. It doesn't matter if the transfer paper overlaps the existing numbers.... they can't be taken off or put back on to the paper.
The same is done with the insignia, in this case the Grossdeutchland helment and German cross.
As I have been putting these on a rough surface I used a cotton bud to directly press down onto the decal (don't rub, just press) to make sure that the flexible ink stick to all the ridges and valleys of the zimmeritt.
There you are! the finished article! no gloss or matt coats, no setting softening solution... 20 minutes work and the NUMBERS AND INSIGNIS ARE DONE AND THERE TO STAY!
here's a close up of a ongoing build which is Amusing Hobby's Lowe as an alternative timeline build in a slightly different Afrika Corps Scheme!
Panzerkampwagen Lowe (Lion) Schwerepanzerbatallion 533, Libiya 1943 Operation "Desert Moon"
slight breaks and alignment are my own mistakes but at a distance can't be noticed and add to the illusion that the numbers etc were put on in the field. The transfers are put on directly onto Vallejo model air with no clear varnishing at all.