Hi Rochie. You can use decal solutions as mentioned, but first, even without these, it's best to have a gloss finish. If you follow the painting guide I sent, use the same technique to brush on Johnson's Klear (Future in the US and other places). One coat might do it, but two are better. When this is TOTALLY dry and set, then use the decal solution. You can use Humbrol DecalFix, but I find it a little bit too 'sticky', and it can leave a deposit which is sometimes hard to remove. It's better to use Microsol and / or Microset. If you can't get it locally, then mail order, such as modelhobbies .co.uk will supply it. But, FOLLOW THE INSTRUCTIONS, as you might be alarmed when, or if, you see the decals crinkle etc! That's normal! When the decals are totaly set, coat on the desired finish varnish, eg. matt, gloss or whatever. For brushing, I use Games Workshop acrylic, or Humbrol acrylic matt. You can thin these with water, or even Klear, and add, if required, a spot or two of Tamiya flat base. As this 'varnish' is thin, if brushed correctly with a wider (No 3 or No4) brush, it should dry without brush marks.
You can also use Micrsol/set to reposition and repair cracked or broken decals, then seal them as above.