Good afternoon
In the last month I've done several tests and I want to share with you my journey!
I have done several samples and I've used S120 and Marbocote HP1000N (same product of HP2002) sealers.
S120 Easy Composite
S120 cannot be applied directly to 3D printed mould because it doesn't stick to it.
I've coated the samples with XTC-3D resin and sand it with 1200 grit sandpaper and I've airbrushed the sample with S120. The result is a very good gloss finished surface. I didn't use this method on my mould because I don't have enough S120 for all my moulds but I think the result I would get will be OK for my purpose.
HP1000N (or HP2002) Marbocote
I have made a sample by spraying the HP1000N directly on ABS. I've sanded the sample before apply the product.
Here the result:


The gloss finish is OK but I was worried about the strenght: after one manufacturing test I have seen that the sample surface has the footprint of the carbon fiber.
So I've decided to use the same method I've used with S120: sand the sample, apply the XTC-3D resin, sand it and apply the HP1000N with an airbrush.
The way to apply the HP1000N is tricky: if you apply 3 or 4 thin coats and the last one very wet you don't get the same gloss finish if you apply only one wet coat.
Here a comparison picture of the 2 methods:

The mould on the bottom side is the one with 3 / 4 light coats + 1 wet coat; it's clearly matte compare to the other one.
The problem with the gloss finish mould is that the HP1000N starts to fade away after 4 or 5 usage. So I think the quantity I've applied is not enough or it has not stick to the mould properly.
Here the "damaged" mould:

I've tried to apply a second coat after a first wet one but the surface had some "hazy areas" so I've stopped to do that.
I'm not sure how I can proceed now: I'm tempted to use the mould to get a resin one like Hanaldo wrote but I would like to explore more these 3D moulds but it's not easy as it looks.
I'm printing a new PLA mould right now and I'm tempted to try to apply the HP1000N directly on it.
I'm afraid that it can be damaged immediately as I've seen on the ABS sample. I'm pressing the moulds with 200Kg and I'm not sure if I can reduce it.
I've just read about using 2K clear coat spray to seal the mould. Maybe if I brush my mould with some resin and then spray a 2K clear coat I can get a gloss and strong finish. Hanaldo wrote that I can get a better gloss finish compare to the one I'm getting by using Marbocote HP1000N.
As usual your comments and advices are more than welcome!!
Cheers
Matteo