...to wash or not/to prime or not.

Tamiya's kits are not known for problems regarding mold release agent on the parts during the build. That being said, you may have had that, or more likely some other surface contaminant from handling after you washed the parts. Tamiya paints are well known for being difficult to hand brush - it could have been that as well.

As Stik says, Tamiya paints are not easy to brush on, especially un thinned. When you brush it on and then brush over it again, you are likely to take as much off as put some on.

As previously mentioned, Tamiya paints don't brush well unthinned. I have heard that when thinned they behave better with a brush. I'm personally not a fan of acrylics but the Vallejo paints do brush out very well. I have also found adhesion problems with acrylics over unprimed surfaces.

I only wash resin parts but you can also wipe the model down before painting with rubbing alcohol.

Neither washing nor priming are bad ideas. On some kits they are necessary,   on others you can get by without either. Personally I seldom wash styrene kits, but always prime. For resin I always wash and prime.

Let's face it, it is a lot less work to wash or prime parts than to remove bad paint from them.

To me this definitely sounds like its the paint not the model.

I'll double down on what others have said about hand brushing Tamiya paint. I've had it do this to me on models that had been primed. It just kinda spreads around like it was butter!

You might also try some Duplicolor adhesion promoter.

before now I have used liquid detergent on Tamiya models and in most cases I find the paint finish is more than acceptable.

Let the model drip dry before applying paint also you can sand between coats for a better finish

man that sux, I've never had an issue with either washing before assembly, or not washing, but here since I got back into building, I usually wash the kit parts in hot water mixed with a good squirt of liquid dish soap then I use an old worn out tooth brush to scrub every part (both sides mind you lol) and let air dry.

Your issue with the paint is the first I've ever heard of a Tamiya not sticking

Bear, I never wash styrene parts. I always use a primer before painting (brush and spray) and never had a problem with adhesion. Bish and others are correct that Tamiya paints are better sprayed than brushed. I would say if you really want to paint them, let the completely dry then give it a second coat, never try to paint over it right away or you'll take a lot of it off and have a real mess. There are other paints that take to a brush a lot better, but for spraying Tamiya thats thinned a bit with an acrylic thinner goes on great.

I have never had much faith in Tamiya acrylics ability to bond to plastic. I always prime my models, but even then I try not to mask over Tamiya acrylics- they just pull up easily.

I'd be lying if I claimed that I wash my plastic kits. Resin, yes always.

I agree with what's been said. When I decided to get back into modeling last year, I bought a bunch of Tamiya acrylics. I would have prefered using enamels, but I'm too impatient to wait for them to dry. Enamels are definitely preferable though. I always prime before painting. I just take the entire kit and hit it with a light coat of Tamiya primer spray while still on the sprues. Then I just cut the pieces off as I need them and do the trimming, sanding, and so on before assembling. It really helps adhesion, and I don't need to worry about priming the areas of the pieces I worked on. Anything that would have been there to interfere with adhesion is removed in the sanding, etc. But whatever is not primed or scuffed up somehow is nearly impossible to cover with Tamiya hand brushed acrylics. The Tamiya is great for airbrushing, though.

The other problem I ran into in trying to brush on the Tamiya acrylics is they dry too fast and leave brush marks everywhere. Deposit it with the brush in one stroke and leave it alone is about all you can do. Because I'm not that good of a painter, I bought a bunch of Vallejo for brushing, and fill in the color gaps with Model Master, now. Of course, with my approach, if I think I will need to do some touch up with a brush after I lay down the base coat, I end up using an air brush to spray on a Vallejo or Model Master base coat so I have good color match. I'm afraid I have a bunch of Tamiya paints I'll never use!

I just read somewhere in the forums about assembling the intakes, then dipping the entire assemblies in a bucket of white household enamel paint, then letting 'em dry until you're ready to put them in the model. Sorry I don't remember which thread I saw this in, but it was in the last couple of days. I've never tried it, but it sure sounds like a great idea once you get the paint adherence issue straightened out. As far as that goes, I always wipe down any surfaces I'm about to paint with alcohol (denatured, if at all possible!) on a clean cloth or paper towel, letting it dry a few minutes, then painting. Never had an issue with that procedure.