You can plus a weapon or pieces of armor in this way up to a maximum of +15
Upgrading Weapons & Armor: aion gold Weapons & Armor Upgrades (Under construction 29 Sep 09)
You can also plus an item by using a Bonus Book of Melding with a cover for the item. The Bonus Book of Melding gives anywhere from +1 to +3, and lasts for 30 days, you can plus a weapon or pieces of armor in this way up to a maximum of +15.
If you perchance need to sell the weapon or armor that used the Bonus Book of Melding, then you simply buy a Book of Unmelding to separate the cover and the item.
You can not sell or trade any item that has the bonus book of meld on it. Bonu Book of Melds that are opened is not trade able
You can use a Platinum refining stone with its +1 to +3 upgrade to the armor or weapon that is not covered, and the effects last for 24 hours real time.
With the refining stone it is possible to get +16, platinum refining stones that are opened or not, are trade able.
You can also upgrade weapons and armor, from level 1 - 73 for armor and Level 1-69 for weapons, with the use of Blood Seals. These add bonuses to different abilities such as dexterity, evasion, mana, health, defense, etc.