
The
Sash skins can go up to +25

The minimum level to use a sash skin is
105 
You can add up to
5 bonus to your sash skin and those bonus are
not removable 
You can switch the bonus that you choose with
Sash Skin Switcher
You can use
Improvement Scroll (Sash Skin) to upgrade your sash, the sash can downgrade

You can use
Improvement Stone (Sash Skin) to upgrade your sash, the sash cannot downgrade

From +0 to +9 your sash skin can have a limit of 1 bonus and that extra only goes up to 1 value (table below)

From +10 to +14 your sash skin can have a limit of 2 bonus and those extras go up to 2 value (table below)

From +15 to +19 your sash skin can have a limit of 3 bonus and those extras go up to 3 value (table below)

From +20 to +24 your sash skin can have a limit of 4 bonus and those extras go up to 4 value (table below)

At +25 your sash skin can have a limit of 5 bonus (Max) and those extras go up to 5 value (table below)