best flat coat in a rattle can?

Testors Dullcoat is my goto.  I have used others, but I get my best results with Dullcoat.

Yep, dullcoat is the best. Agree with you about the cost though.

I stopped using the Testors rattle can clear coats because with age they tend to interact with decal set and and solvent solutions leaving "tide marks" and also tend to discolor with an amber tone. It takes years to be noticeable, but it becomes more pronounced with time.

I have never had dullcoat nor glosscoat change colors with age, however, their whites- both gloss and age used to.  I hear they fixed that, but as it used to take years to yellow, I am not convinced.  I just try to keep the models away from direct sunlight now.

Well, I sure cant say why mine did. I know that it is not due to nicotine as all of the places that I lived are smoke free, nobody in my family smokes. All of the kits were built 1990-1994 and of those that changed hue over the years, almost all of them were never displayed in direct sunlight. At least not before the amber hue became noticeable. And the "tide marks" interaction with Micro Sol/Micro Set is a whole different ball of wax. Perhaps both are due to local climate and air content. I just put it out there to say it can happen in the long term with these particular products.

Do you have a Michael's store near you. I get my Dullcote at Michael's with a 40 to 55% discount coupon.