You may struggle with decals on a matt surface anyway. Fairly std practice waterslide decals should be on satin or gloss surface and varnished over after. Otherwise they don't take (oodles of info out there on this). I use the decal solvent promoters / dissolvers also. Think it's from micromark (not near it now to check) > a world of difference with to without, it dissolves the backing and the decal conforms and sticks like the provervial. After which you still need a coat of varnish to fix / seal. Can't help you much with varnish apart from vallejo polyurethane. I still try to keep using oil based tinlets otherwise as much as I can, top varnish of polyurethane, just where it needs it usually.