How long to wait before you start to apply decals

In general, 24 hours should be plenty. That being said, Gloss paints may take longer to dry and if you're in a humid area it may take even longer. IMHO 24 hours is a good rule of thumb.

 As to your Humbrol issue, that could be caused by a number of things, maybe Humbrol's mix just isn't quite matte or you didn't quite get it all mixed up right, so it may stay glossy or semi-gloss even when dry. If you've given it a couple of days already, you should be fine with applying your decals.

Well I got the thing decaled but now The color underneath is showing thru, and I have no extra decals. Question is can I paint over the decal to cover up the color showing thru from the underside?  Would painting overthem do any harm to them?. The decal in question are white stripes.

I usually wait a few days or more, even a week to allow gloss finishes to dry completly. Humidity and tempurature can factor as well.

White decals can be a pain with colors showing through, even clothing in white as well, if you've ever noticed. I've seen some decals that go on in two parts with a base decal that is allowed to set completley, then the finish one going on over that. I've heard of modelers who will paint a base under a white decal. I've never done this as it sounds like a pain.

The decals I bought for my P-51 have two options for a blue checkerboard pattern on the tail. It has a decal with blue and white but also includes a clear and blue checker one that you need to paint the area white before applying the decal. They recommend tha last technique becase of the white issue.

Some modelers will buy two sets of decals and apply one over the other....

Painting over them won't really harm the decals but you've got to take care with brushing for it not to stand out as a repair. You might try getting some white decal film and using that over your current areas.

 

 

FYI Humbrol matt paints MUST be stirred very very thoroughly and often during use. I found this out the hard way years ago when I had the problem you describe. Its worth the effort they are very good paints for certain things.