OK, now I understand the problem-- Tamiya primer, with MM enamel paints. Or are you using MM Acrylic? -- if so, that's your main problem. If they are MM enamels, you shouldn't have any problem. Firstly, it's better to spray primers, rather than brush painting. The reason is they tend to have more uniform coverage when sprayed. Brush painting primers leads to paint build up in some areas, which dries at varying rates, without a uniform finish. This produces "hills and valleys" in the paint, which has a tendency to cause overlying coats to pool or be "repelled". In addition, Tamiya paints are Acrylic lacquers, and they can have an effect on enamels. Acrylic lacquers also tend to dry with a more glossy or finely pigmented finish that will actually repel the carrier in some enamels, which MM paints are. If brush painting is the only way you have for painting, accurate thinning is definitely required-- with the proprietary thinners. In this case, Tamiya Lacquer Thinner for the Tamiya primer, and Testor's enamel thinner for the MM paints. Using a wider brush for full coverage will also help. For the primer, you probably need at least two or three "thin" coats, allowing at least 24 hours between coats to dry. Always brush in the same direction. Then allow an additional 24 hours before applying any enamels (MM) paint, and again, starting with a thin coat of about 50:50 ratio of paint to thinner. Avoid scrubbing. All paints need to be thoroughly mixed. I think it's a myth that Tamiya paints are hard to brush-- out of the bottle yes, but properly thinned mixtures are as easy as any other paints. Really the skill here is to find the correct ratios for mixing. For Tamiya paints, because they are an Acrylic Lacquer, they tend to dry much faster than enamels, which is why many folks consider them difficult to brush paint-- the key is to add paint and thinner into the mix to keep the paint viable over time. And to be patient with the drying process. And remember, paint coverage using dissimilar thinners and carriers can often be problematic. It might be advisable to use a "barrier" coat of clear to prevent one type of thinner interacting with another type of paint. I also recommend a good spray primer. Krylon (which owns Testor's by the way), or another enamel brand might be better. I've used MM paints for years with no problem, and I've used Tamiya for years with no problem, and I've used the two together on occasion with no problem, but the key is proper thinning and patience. I recommend mixing paints in containers outside the bottle too-- get a deep-dish pallet or have several mixing containers on hand.
VR, Russ