clear coating AFV's before decaling

Use future but spray it undiluted in thin coats and you won't get a heavy Build Up. Make sure you cover the kit afterward just to make sure no dust settles on it. Sprayed Future does not take too long to dry.

I use Future except i just apply it to where the decal is going to be. After the decal is on , cover with a dull coat.

No one correct way to do this as everyone develops their own style to handle it. I've never used Future, so can't comment, but have achieved good results using brushed on gloss coat for the decal location followed by a second brush coat when the decals are dry and then an overall flat/matte coat to seal in decals and paint prior to weathering.

If you have an old kit you can always experiment on it first so as not to interfere with your finished product and achieve the result you want that way. I keep all my unused decals from previous kits and used them to work out different ways of handling different decal types. Comes in very handy.