Weapon and Armor Expertise

If I level a character to say 52 then delevel him to 25 will he still be able to use B Grade weapon/armor without penalty?

you can wear it, but you'll have a grade penalty. you'll miss a lot and with an armor you'll be so slow.

then you can't:p

Nope you can't

Grade masteries are now lost immediately when you delevel below their minimum level. All other sills bought with SP are kept until you go more than ten levels below the level you got them at. (This is per level of the skill, not just the first or highest level of the skill youu have.)

Short answer to your question: No. At 51 you would lose the B grade mastery.