Dave, I use regular low-odor mineral spirits, or your simple generic "Paint thinner" from the local hardware store. NO, it will NOT harm the Tamiya acrylics.
Other acrylics MAY or may not be more "fragile"; I also don't seal my MM acrylics, and never have a problem with them.
I wish I knew where this rumor/disinformation came from about acrylics being so fragile? I think I remember hearing something about them used-to being fragile in the past, but nowadays they're the best option for painting in my opinion.
Some guys still use enamels, and if you do ,then you DO have to seal them in. But nowadays, in Auto Modeling, where I am posting lately (building a Modified Stocker) enamels used to be 'THE" paint. But lately, lacquers have taken over, and Tamiya's acrylics/lacquers are really amazing paints for laying down beautiful color coats straight from the can--they even have Armor colors in cans now for the airbrush-challenged.
My advice? Ditch the whole concept of sealing in your paint coat if you're using Tamiya or MM acrylics. If you want to put a spot of gloss under the decals, then do that--I do that too, sometimes. And some guys prefer doing washes over gloss; I do not. But it's still a viable technique that you should not dismiss. However, you CAN do it over simple base coats. The difference is that the flat base coat willl "grab" the color more than a gloss coat will.Just remember, do the filters and drybrushing over a flat coat--you can actually see the results more accurately as well over a flat coat.
Good luck, and I'll try to remmeber to check into this build form time to time between plumbing my Chevy small block! ![Blush [:I]](/emoticons/icon_smile_blush.gif)