Okay, here is what I encopuntered step by step before I ask my quesdtions:

1. Plastic model (Bronco) built up ready to paint.
2. Primed with Vallejo Grey Primer (thinned down probably 1:1 because of clogging issues in the past. Result: An even enough layer.
3. Let it sit for a few days.
4. Sprayed some AMMO (Mig) Dark Grey over it in several thin layers, quite heavily thinned down (2:1) because of clogging and blotching issues in the past. Result: Okay enough layer of paint, a very, very little grainy but still quite okay.
5. Let it sit for 24 hours.
6. Applyed a thin enamel filter (AMMO by Mig) over the whole thing.
7. Let it sit for 48 hours.
8. Applied two thin coats of Vallejo Gloss Varnish thinned down a bit (1:2) where the decals are supposed to go (only locally, not the whole model).
9. Let it sit for 24 hours.
10 Applied the decals and put a drop of Tamiya Mark Fit (extra strong) on it. Result: The Gloss peeled of in one big gummy peel (like sun burnt skin after the skin regenerated )

So my question is: What the ? Was it the MIG color that didn't provide enough stickyness for the gloss layer? Was it the Vallejo Acrylics Gloss paint that is crap? Was it the way I applied it/thinned it that was crap? Is the Tamiya Mark Fit too crazy for Non-Laquer/Non-Tamiya-Acrylics?

My thoughts are: Maybe I should have painted wider areas in gloss in order not to let the Mark Fit crawl under the coat at the edges? Or should I not have thinned the paints (gloss and dark grey) so much (to provide both a better base for the gloss to grip and a sturdier gloss coat? Or should I have used Tamiya paints in the first place (allowing the Mark Fit to do its work without attacking the gloss coat)? I don't know and would have to try all sorts of set ups to find a solution... but maybe some of you could give me a hint or even add their own experiences...

Not sure of specific cause, as I haven't experienced this problem.

But, I always spray my gloss clear over the whole model. Then apply decals. I use the same Tamiya Mark-Fit softener. After a few days drying. I spray a coat of flat or ultra matte clear.

I generally use the AK varnish.

What are you using to reduce your paint? Tamiya gloss clear is excellent(reduced 1:1, with Mr. Hobby Leveling Thinner). If you want to go the acrylic route, I suggest Johnson's Future. Future should not be reduced.

For the AMMO colors I use their thinner, the same goes with Vallejo (meaning with AV I use their AV Thinner).

It came to my mind that maybe my order of things could be the problem since I added a layer of Enamel filter between the AMMO Dark Grey and the Vallejo Gloss Coat... Could that have reduced the ability to stick for the gloss coat? Rephrased: The Mark Fit attacked the enamal filter at the edges of the gloss coat and made it peel?

I have heard that you should apply enamel over acrylic. Since Tamiya acryls are not true acrylics, their gloss clear may be the way to go.

I believe the problem came from the enamel wash preventing correct adhesion of your acrylic gloss. I highly recommend that you begin your washes or filters after decal application so you don’t have problems with enamel paints taking extended times to fully cure.

Additionally, once paints are thinned to the level of a wash, adhesion is iffy at best. For most this isn’t a big deal, but if you need to do additional layers over that layer, it becomes a very serious problem.

Just two cents from past experience,
Paul H

I would certainly give it a try... but my paint cabinet is filling up like crazy and the more I get, the more problems occur... should stick to good old Tamiya and Vallejo... never had any real problems back then when they were the only ones I was using

A rule of thumb I was told long ago is to always apply your clear coat to the entire model before applying any type of wash or weathering effect. This way your paint job is protected.

I know I might have taken a shortcut with only spraying the spots for the decals, but the underlying paint job isn't the problem here, it stays in place just fine. The problem is the clear coat itself, that peels off when Mark Fit is applied...

For me it's not a shortcut I always do that and I've never had any problems, I do a coat of clear on the whole piece only if the base colors are enamel, but as softner I use MicroSol and MicroSet never tried Tamiya products

You may want to consider applying a heavier layer of gloss.

First off I don't do local applications of varnish because I have in the past and it was noticeable. I had a similar problem putting solvaset decal liquid over Tamiya clear. I tested out some different decal liquids and found that I had no problem with MicroScale decal liquid. When I paint a model I usually apply the same exact application of finishing the model to a couple scrap squares of sheet styrene just incase I want to test something out and see how it would effect the model...you could also do the same thing on the underside of the model that won't show...experiment

you could also try out dry transfers which go on easy and all you have to do is cover the transfers with varnish once applied

I use Walther's and have never had a problem placing decals over Tamiya acryls or true acrylics.