Trying to make all down strokes sound exactly like upstrokes is impossible and counter intuitive. They're different and need to sound different.
I agree and disagree. Jody Fisher (head of the Los Angeles Music Academy guitar department and author of over 30 books) states that one of the important things in the practice of alt picking is to try and get both strokes to sound as similar as possible. Of course they normally sound a bit different due to pick angle (mostly), but I agree with the idea of being able to make them indistinguishable. By choice. You should be able to control every aspect of your sound by choice.
I think what you're getting at is more accenting and the fact that we tend to accentuate downstrokes, because for most, it is the more natural of the motions, making it hard for rhythmically untrained guitarplayers to start on upstrokes, or work with patterns that reverse/syncopate accents.
On the original topic, I, too have struggled with clean alternate picking for a very, very long time. More than I care to admit, let's just say that other guitar players' entire carees have been shorter
In the past 18 months I've been on a quest for the picking technique (again, after nearly giving up on the whole thing a couple of years ago).
What I have found is this: There are very many very minute adjustments to be made, in order to find one's own perfect picking technique. And slowing down does not always give you all the answers, because it tends to mask problems, that occur at higher speeds. Of course once you have found out WHAT to do, you need to practice slowly. But: The thing is, when you compare the picking styles of Paul Gilbert, Yngwie Malmsteen and Steve Morse, each one has phenomenal picking technique, but all three LOOK very distinctly different from each other. I spent a long time obsessing over the details of my motion, practicing the wrist movement isolated, with my hand on a table and eliminating all extraneous movement. Watching my guitar teacher closely and seeing which muscles he engages (because they pop out very visibly) also helped. I tried to really focus on the wrist movement, because a lot of my picking came from my forearm, which made my pick bounce around (in relation to height over the string), and it was several "mini-motions" of my fingers, arm and wrist counteracting each other. It was a very grueling process, and really a lot of reprogramming muscle memory was necessary, to get up to where I am now.
All those exercises are great, but if you don't know how to physically do them "right", you're not necessarily going to learn it from scale exercises. For me, like I said, a lot of very, very detailed observation was necessary.
so long
Andreas aka Ace