Final armour coat

Seeing as this is your first post Poipey ... welcome to FSM.

I usually use a matt finish such as Testor's Dullcote after all the decals have been placed on but before I start doing any weathering.

I agree with Robert, but I will also flat coat (right out of the Testor's spray can ) after weathering. I find this process locks in drybrushing, chalk application, etc. It's advantageous when handling the model, or having to dust it in the future.
Gip Winecoff

Thanks for the tips. However, I've been told that when putting decals onto a matt paint they sometimes have difficulty sticking because of the rougher surface. Would it be advantagous to put a gloss finish on, put on the decals and then finish of with a dull finish.
Poipey

Absolutely .... after I have painted my base coats and am ready to apply decals, I will brush Future Floor Polish onto the areas where the decals are going to go. Many modellers apply Future to the entire model to give it more consistency. Then I will apply the decals the Micro Sol and Micro Set solutions and follow this up with 2 -4 light coats of Dull-cote to matt and seal the entire model. If you are going to do washes you would want to do them after the Gloss finish and before the matt finish but I would do all other weathering after the matt finish is applied.